See the Celery docs.
Default is four, which could explain why tasks don't start concurrently when there are available workers, as one worker with a long running task may have grabbed the next three or four tasks too.
Issue #330 was marked as a duplicate of this issue.
This was fixed in 4fd7925 back in December 2015!
Confirmed fixed #327 so added to changes/CHANGES
→ <<cset 7f7b18f25ed2>>