No single access point can handle thousands of users. Furthermore, this question is way deeper and more complicated than a simple, or even long answer here can cover, but at least I'll try to explain why:
- on a large number of users, the main problem is shared media that gets flooded with great number of transmitters and receivers: imagine several hundreds of people shouting at one person, and that person is your access point. Even if they would speak normally or even whisper, there's a loudness/distance threshold, and no communication is possible after crossing it
- so the main approach is to split the area into a set of areas, so every one will be handled by one access point
- there's great number of considerations when calculating how many access point you will need
- there's a great number of considerations when calculating the power level for every one of these access points
- there's a great number of considerations when choosing access points and their antennas - including, but not limited the ability for clients to roam, sectors coverage and signal amplification.
As an example of such considerations I can recommend a white paper from Cisco, just to understand how complex it can be: Cisco Wireless LAN Design Guide for High Density Clients in High Education
However, I suppose that without proper experience you just cannot ensure your first wireless installation will handle thousand of wireless users. You don't need a stress test for that. Furthermore, there's no such stress test that would simulate thousands (or hundreds) of transmitters in one area.