Index: test/dosep.py
===================================================================
--- test/dosep.py
+++ test/dosep.py
@@ -1350,6 +1350,11 @@
                 runner_strategies_by_name.keys()))
     test_runner_func = runner_strategies_by_name[test_runner_name]
 
+    # On Windows, Python uses CRT with a low limit on the number of open files.
+    # Capping the number of threads avoids "OSError: too many open files."
+    if os.name == 'nt' and num_threads > 32:
+        num_threads = 32
+
     summary_results = walk_and_invoke(
         test_directory, test_subdir, dotest_argv,
         num_threads, test_runner_func)