Fix normalize_num_threads for macos

imops/utils.py

@@ -1,4 +1,5 @@
 import os
+import platform
 from contextlib import contextmanager
 from itertools import permutations
 from typing import Callable, Optional, Sequence, Tuple, Union
@@ -53,7 +54,7 @@ def normalize_num_threads(num_threads: int, backend: Backend, warn_stacklevel: i
     env_num_threads = os.environ.get(env_num_threads_var_name, '').strip()
     env_num_threads = int(env_num_threads) if env_num_threads else None
     # TODO: maybe let user set the absolute maximum number of threads?
-    num_available_cpus = len(os.sched_getaffinity(0))
+    num_available_cpus = os.cpu_count() if platform.system() == 'Darwin' else len(os.sched_getaffinity(0))
 
     max_num_threads = min(filter(bool, [IMOPS_NUM_THREADS, env_num_threads, num_available_cpus]))