Hello Son Nguyen
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.