EMS sends you an email when you are running out of licenses. Additionally, a log entry is entered when a client is refused connection due to unavailable licenses.
I understand from it that you can't register more clients (devices) when you reach the max number of registered clients.
Ref: https://docs.fortinet.com/document/forticlient/7.4.0/licensing-guide/715745/chromebook-licenses
The page is about Chromebook but I guess the bottom note is also applicable to other clients.
On the other hand, I may understand from your first post that you may have a user-based license, right? In that case I don't know, I'm only used to device-based licensing, and my above statement is regarding device-based, not user-based.