exp run: jobs should default to 1 (#5785)

dvc/command/experiments.py

@@ -1213,6 +1213,7 @@ def _add_run_common(parser):
         "-j",
         "--jobs",
         type=int,
+        default=1,
         help="Run the specified number of experiments at a time in parallel.",
         metavar="<number>",
     )

tests/unit/command/test_experiments.py

@@ -93,7 +93,7 @@ def test_experiments_run(dvc, scm, mocker):
         "name": None,
         "queue": False,
         "run_all": False,
-        "jobs": None,
+        "jobs": 1,
         "tmp_dir": False,
         "checkpoint_resume": None,
         "reset": False,