AP6521 have a limit of 128 local radius user. All other APs limited by 256 user in the database.
This user can be EAP-MSCHAPv2 or Captive Portal user.
If you use local radius with LDAP as database, there is no limit. Also for PEAP-TLS or TLS there is "no limit", because in both cases the database is not local. Same, if you use central VX or NX. The limit is for the user database mapped on an AP.
From my own experience, for example AP7502 you can create 2 simultaneous SSH sessions but when creating the 3 or 4 connection is prohibited with out of RAM message. When with AP6532 or the newest AP models don't had that problems. So only limitation is RAM.
If you searching for a management configuration limitation of device, I can give you a direction to see in to the config file in the internal memory of your device. Update me if you want to know the file path.