From what I know there are no upper limits in the 802.11 protocol. There are software limits in some of the AP's I've worked with. Aerohive usually have 100 concurrent stations on most AP's.
The way wifi works is that only one client can talk to the AP at any given time. So the more stations, less time to talk for every each one of them.
My guess is that whatever AP you have, the upper limit will be the when the amount of RAM runs out, or the conn-track table is full and hogs the CPU too much.