Use the process CPU count to determine how many children to create (#8268)

script/helpers.py

@@ -188,3 +188,14 @@ def get_binary(name: str, version: str) -> str:
             """
         )
         raise
+
+
+def get_usable_cpu_count() -> int:
+    """Return the number of CPUs that can be used for processes.
+
+    On Python 3.13+ this is the number of CPUs that can be used for processes.
+    On older Python versions this is the number of CPUs.
+    """
+    return (
+        os.process_cpu_count() if hasattr(os, "process_cpu_count") else os.cpu_count()
+    )