diff --git a/madoop/__main__.py b/madoop/__main__.py
index 713604c..90409c2 100644
--- a/madoop/__main__.py
+++ b/madoop/__main__.py
@@ -34,6 +34,10 @@ def main():
         '-v', '--verbose', action='count', default=0,
         help="verbose output"
     )
+    optional_args.add_argument(
+        '-numReduceTasks', dest='num_reducers', default=4,
+        help="max number of reducers"
+    )
     required_args = parser.add_argument_group('required arguments')
     required_args.add_argument('-input', dest='input', required=True)
     required_args.add_argument('-output', dest='output', required=True)
@@ -60,6 +64,7 @@ def main():
             output_dir=args.output,
             map_exe=args.mapper,
             reduce_exe=args.reducer,
+            num_reducers=int(args.num_reducers)
         )
     except MadoopError as err:
         sys.exit(f"Error: {err}")
diff --git a/madoop/mapreduce.py b/madoop/mapreduce.py
index 26dfa38..6afb58b 100644
--- a/madoop/mapreduce.py
+++ b/madoop/mapreduce.py
@@ -19,14 +19,13 @@
 MAX_INPUT_SPLIT_SIZE = 2**20  # 1 MB
 
 # The number of reducers is dynamically determined by the number of unique keys
-# but will not be more than MAX_NUM_REDUCE
-MAX_NUM_REDUCE = 4
+# but will not be more than num_reducers
 
 # Madoop logger
 LOGGER = logging.getLogger("madoop")
 
 
-def mapreduce(input_path, output_dir, map_exe, reduce_exe):
+def mapreduce(input_path, output_dir, map_exe, reduce_exe, num_reducers):
     """Madoop API."""
     # Do not clobber existing output directory
     output_dir = pathlib.Path(output_dir)
@@ -74,6 +73,7 @@ def mapreduce(input_path, output_dir, map_exe, reduce_exe):
         group_stage(
             input_dir=map_output_dir,
             output_dir=reduce_input_dir,
+            num_reducers=num_reducers
         )
 
         # Run the reducing stage
@@ -240,14 +240,19 @@ def keyhash(key):
     return int(hexdigest, base=16)
 
 
-def partition_keys(inpath, outpaths, input_keys_stats, output_keys_stats):
+def partition_keys(
+        inpath,
+        outpaths,
+        input_keys_stats,
+        output_keys_stats,
+        num_reducers):
     """Allocate lines of inpath among outpaths using hash of key.
 
     Update the data structures provided by the caller input_keys_stats and
     output_keys_stats.  Both map a filename to a set of of keys.
 
     """
-    assert len(outpaths) == MAX_NUM_REDUCE
+    assert len(outpaths) == num_reducers
     outparent = outpaths[0].parent
     assert all(i.parent == outparent for i in outpaths)
     with contextlib.ExitStack() as stack:
@@ -255,13 +260,13 @@ def partition_keys(inpath, outpaths, input_keys_stats, output_keys_stats):
         for line in stack.enter_context(inpath.open()):
             key = line.partition('\t')[0]
             input_keys_stats[inpath].add(key)
-            reducer_idx = keyhash(key) % MAX_NUM_REDUCE
+            reducer_idx = keyhash(key) % num_reducers
             outfiles[reducer_idx].write(line)
             outpath = outpaths[reducer_idx]
             output_keys_stats[outpath].add(key)
 
 
-def group_stage(input_dir, output_dir):
+def group_stage(input_dir, output_dir, num_reducers):
     """Run group stage.
 
     Process each mapper output file, allocating lines to grouper output files
@@ -269,8 +274,9 @@ def group_stage(input_dir, output_dir):
 
     """
     # Compute output filenames
+    LOGGER.debug("%s reducers", num_reducers)
     outpaths = []
-    for i in range(MAX_NUM_REDUCE):
+    for i in range(num_reducers):
         outpaths.append(output_dir/part_filename(i))
 
     # Track keyspace stats, map filename -> set of keys
@@ -279,7 +285,8 @@ def group_stage(input_dir, output_dir):
 
     # Partition input, appending to output files
     for inpath in sorted(input_dir.iterdir()):
-        partition_keys(inpath, outpaths, input_keys_stats, output_keys_stats)
+        partition_keys(inpath, outpaths, input_keys_stats,
+                       output_keys_stats, num_reducers)
 
     # Log input keyspace stats
     all_input_keys = set()
diff --git a/tests/test_api.py b/tests/test_api.py
index a75d711..8754970 100644
--- a/tests/test_api.py
+++ b/tests/test_api.py
@@ -13,6 +13,7 @@ def test_simple(tmpdir):
             output_dir="output",
             map_exe=TESTDATA_DIR/"word_count/map.py",
             reduce_exe=TESTDATA_DIR/"word_count/reduce.py",
+            num_reducers=4
         )
     utils.assert_dirs_eq(
         TESTDATA_DIR/"word_count/correct/output",
@@ -20,6 +21,22 @@ def test_simple(tmpdir):
     )
 
 
+def test_2_reducers(tmpdir):
+    """Run a simple MapReduce job with 2 reducers."""
+    with tmpdir.as_cwd():
+        madoop.mapreduce(
+            input_path=TESTDATA_DIR/"word_count/input",
+            output_dir="output",
+            map_exe=TESTDATA_DIR/"word_count/map.py",
+            reduce_exe=TESTDATA_DIR/"word_count/reduce.py",
+            num_reducers=2
+        )
+    utils.assert_dirs_eq(
+        TESTDATA_DIR/"word_count/correct/output-2-reducers",
+        tmpdir/"output",
+    )
+
+
 def test_bash_executable(tmpdir):
     """Run a MapReduce job written in Bash."""
     with tmpdir.as_cwd():
@@ -28,6 +45,7 @@ def test_bash_executable(tmpdir):
             output_dir="output",
             map_exe=TESTDATA_DIR/"word_count/map.sh",
             reduce_exe=TESTDATA_DIR/"word_count/reduce.sh",
+            num_reducers=4
         )
     utils.assert_dirs_eq(
         TESTDATA_DIR/"word_count/correct/output",
@@ -43,6 +61,7 @@ def test_bad_map_exe(tmpdir):
             output_dir="output",
             map_exe=TESTDATA_DIR/"word_count/map_invalid.py",
             reduce_exe=TESTDATA_DIR/"word_count/reduce.py",
+            num_reducers=4
         )
 
 
@@ -54,6 +73,7 @@ def test_missing_shebang(tmpdir):
             output_dir="output",
             map_exe=TESTDATA_DIR/"word_count/map.py",
             reduce_exe=TESTDATA_DIR/"word_count/reduce_invalid.py",
+            num_reducers=4
         )
 
 
@@ -65,6 +85,7 @@ def test_empty_inputs(tmpdir):
             output_dir="output",
             map_exe=TESTDATA_DIR/"word_count/map.py",
             reduce_exe=TESTDATA_DIR/"word_count/reduce.py",
+            num_reducers=4,
         )
     utils.assert_dirs_eq(
         TESTDATA_DIR/"word_count/correct/output",
@@ -80,6 +101,7 @@ def test_single_input_file(tmpdir):
             output_dir="output",
             map_exe=TESTDATA_DIR/"word_count/map.py",
             reduce_exe=TESTDATA_DIR/"word_count/reduce.py",
+            num_reducers=4,
         )
     utils.assert_dirs_eq(
         TESTDATA_DIR/"word_count/correct/output",
@@ -97,6 +119,7 @@ def test_ignores_subdirs(tmpdir):
             output_dir="output",
             map_exe=TESTDATA_DIR/"word_count/map.py",
             reduce_exe=TESTDATA_DIR/"word_count/reduce.py",
+            num_reducers=4
         )
     utils.assert_dirs_eq(
         TESTDATA_DIR/"word_count/correct/output",
diff --git a/tests/test_cli.py b/tests/test_cli.py
index 0d6c7f1..dab719e 100644
--- a/tests/test_cli.py
+++ b/tests/test_cli.py
@@ -49,6 +49,27 @@ def test_simple(tmpdir):
     )
 
 
+def test_2_reducers(tmpdir):
+    """Run a simple MapReduce job with 2 reducers."""
+    with tmpdir.as_cwd():
+        subprocess.run(
+            [
+                "madoop",
+                "-input", TESTDATA_DIR/"word_count/input",
+                "-output", "output",
+                "-mapper", TESTDATA_DIR/"word_count/map.py",
+                "-reducer", TESTDATA_DIR/"word_count/reduce.py",
+                "-numReduceTasks", "2",
+            ],
+            stdout=subprocess.PIPE,
+            check=True,
+        )
+    utils.assert_dirs_eq(
+        TESTDATA_DIR/"word_count/correct/output-2-reducers",
+        tmpdir/"output",
+    )
+
+
 def test_verbose(tmpdir):
     """Run a simple MapReduce job and verify the verbose stdout."""
     with tmpdir.as_cwd():
diff --git a/tests/test_stages.py b/tests/test_stages.py
index dad39f7..7ee82d0 100644
--- a/tests/test_stages.py
+++ b/tests/test_stages.py
@@ -23,6 +23,7 @@ def test_group_stage(tmpdir):
     group_stage(
         input_dir=TESTDATA_DIR/"word_count/correct/mapper-output",
         output_dir=Path(tmpdir),
+        num_reducers=4,
     )
     utils.assert_dirs_eq(
         TESTDATA_DIR/"word_count/correct/grouper-output",
@@ -30,6 +31,19 @@ def test_group_stage(tmpdir):
     )
 
 
+def test_group_stage_2_reducers(tmpdir):
+    """Test group stage using word count example with 2 reducers."""
+    group_stage(
+        input_dir=TESTDATA_DIR/"word_count/correct/mapper-output",
+        output_dir=Path(tmpdir),
+        num_reducers=2,
+    )
+    utils.assert_dirs_eq(
+        TESTDATA_DIR/"word_count/correct/grouper-output-2-reducers",
+        tmpdir,
+    )
+
+
 def test_reduce_stage(tmpdir):
     """Test reduce stage using word count example."""
     reduce_stage(
@@ -41,3 +55,16 @@ def test_reduce_stage(tmpdir):
         TESTDATA_DIR/"word_count/correct/reducer-output",
         tmpdir,
     )
+
+
+def test_reduce_stage_2_reducers(tmpdir):
+    """Test reduce stage using word count example with 2 reducers."""
+    reduce_stage(
+        exe=TESTDATA_DIR/"word_count/reduce.py",
+        input_dir=TESTDATA_DIR/"word_count/correct/grouper-output-2-reducers",
+        output_dir=Path(tmpdir),
+    )
+    utils.assert_dirs_eq(
+        TESTDATA_DIR/"word_count/correct/reducer-output-2-reducers",
+        tmpdir,
+    )
diff --git a/tests/testdata/word_count/correct/grouper-output-2-reducers/part-00000 b/tests/testdata/word_count/correct/grouper-output-2-reducers/part-00000
new file mode 100644
index 0000000..48e2fd5
--- /dev/null
+++ b/tests/testdata/word_count/correct/grouper-output-2-reducers/part-00000
@@ -0,0 +1,6 @@
+Bye	1
+Goodbye	1
+Hadoop	1
+Hadoop	1
+World	1
+World	1
diff --git a/tests/testdata/word_count/correct/grouper-output-2-reducers/part-00001 b/tests/testdata/word_count/correct/grouper-output-2-reducers/part-00001
new file mode 100644
index 0000000..f7fef0c
--- /dev/null
+++ b/tests/testdata/word_count/correct/grouper-output-2-reducers/part-00001
@@ -0,0 +1,2 @@
+Hello	1
+Hello	1
diff --git a/tests/testdata/word_count/correct/output-2-reducers/part-00000 b/tests/testdata/word_count/correct/output-2-reducers/part-00000
new file mode 100644
index 0000000..369bdd3
--- /dev/null
+++ b/tests/testdata/word_count/correct/output-2-reducers/part-00000
@@ -0,0 +1,4 @@
+Bye 1
+Goodbye 1
+Hadoop 2
+World 2
diff --git a/tests/testdata/word_count/correct/output-2-reducers/part-00001 b/tests/testdata/word_count/correct/output-2-reducers/part-00001
new file mode 100644
index 0000000..30f4be7
--- /dev/null
+++ b/tests/testdata/word_count/correct/output-2-reducers/part-00001
@@ -0,0 +1 @@
+Hello 2
diff --git a/tests/testdata/word_count/correct/reducer-output-2-reducers/part-00000 b/tests/testdata/word_count/correct/reducer-output-2-reducers/part-00000
new file mode 100644
index 0000000..369bdd3
--- /dev/null
+++ b/tests/testdata/word_count/correct/reducer-output-2-reducers/part-00000
@@ -0,0 +1,4 @@
+Bye 1
+Goodbye 1
+Hadoop 2
+World 2
diff --git a/tests/testdata/word_count/correct/reducer-output-2-reducers/part-00001 b/tests/testdata/word_count/correct/reducer-output-2-reducers/part-00001
new file mode 100644
index 0000000..30f4be7
--- /dev/null
+++ b/tests/testdata/word_count/correct/reducer-output-2-reducers/part-00001
@@ -0,0 +1 @@
+Hello 2