Allow specifying --workers via environment variable

CHANGES.md

@@ -25,6 +25,8 @@
 
 <!-- Changes to how Black can be configured -->
 
+- The `--workers` argument to Black can now be specified via the `BLACK_NUM_WORKERS`
+  environment variable (#3743)
 - `.pytest_cache`, `.ruff_cache` and `.vscode` are now excluded by default (#3691)
 
 ### Packaging

src/black/__init__.py

@@ -373,7 +373,10 @@ def validate_regex(
     "--workers",
     type=click.IntRange(min=1),
     default=None,
-    help="Number of parallel workers [default: number of CPUs in the system]",
+    help=(
+        "Number of parallel workers [default: BLACK_NUM_WORKERS environment variable "
+        "or number of CPUs in the system]"
+    ),
 )
 @click.option(
     "-q",

src/black/concurrency.py

@@ -80,7 +80,8 @@ def reformat_many(
 
     executor: Executor
     if workers is None:
-        workers = os.cpu_count() or 1
+        workers = int(os.environ.get("BLACK_NUM_WORKERS", 0))
+        workers = workers or os.cpu_count() or 1
     if sys.platform == "win32":
         # Work around https://bugs.python.org/issue26903
         workers = min(workers, 60)