set-funcall-async-limit limits the number of asynchronous calls (by funcall-async or funcall-async-list) which can run in parallel. Further asynchronous calls are queued, and when a running call finishes another call starts.
The default limit is 5, which is adequate if funcall-async and/or funcall-async-list are only used occasionally. If you use them often, you may want to increase this limit to between 10 and 30. A larger limit probably does not make sense.
LispWorks User Guide and Reference Manual - 20 Sep 2017