The "back of the napkin" calculation for APs is one gigabitethernet port for each 100 access points. You can bond two together for each controller and monitor the utilization. The "bottleneck" in the WLAN is typically wireless contention, which usually keeps each access point from performing at its theoretical maximum as more and more clients join. The second bottleneck are the resources, like the internet that users are trying to access. The one per every 100 APs typically works well, and you can monitor to see if you would add more. Again, unless your switches are connected by 10gigabit ethernet connectors, you would again have another bottleneck. Wireless traffic is typically bursty and never reaches the sustained levels that people anticipate.