If you go slightly over, I would not expect any issues. If you exceed the user count significantly, expect unexpected behavior as you are running outside the specifications, sizing, and what is being tested.
The easy fix here would be to size your MM to the MM-VA-1K specifications of RAM/CPU/disk, after which it will change to a 1K 'performance' with the 500 devices licenses. You can't exceed the AP/controller count, you can go to the MM-1K platform-limit for clients.