How many WiFi networks can operate concurrently?
You had two parts in the question:


How many devices can be connected simultaneously to a single one network? The limit is when the traffic reaches the saturation level of the channels. It depends on how frequently and how much data the WiFi clients use to communicate. 
Is there a limit to how many different WiFi networks can work properly in the same place? Yes, it can happen fast. Ideally, each Wifi access point should choose a different non-overlapping channel. Ideally, channels 1, 6 or 11 on 2.4GHz. The 5GHz space has more alternatives and bandwidth, thus it can better tolerate the same amount of access points than 2.4GHz in the same physical space.


Below is the channel utilization my Ubiquity access point sees in a highly-populated area.