Symptom:
Prime infrastructure displays the following error:-

?You are approaching the maximum license capacity on controller. Please add a license with greater capacity to this controller.?

Despite of the fact that you have valid base-ap-count permanent license installed and capacity is available.

Conditions:
virtual wireless lan controller with "wlc-virtual"/ base license in use and Prime infrastructure monitoring the virtual wireless lan controller.

virtual wireless lan controller still has enough licenses on it, so more LAPs can join on it.