I'll try to handle crosstalk with software, it shouldn't matter if the data dumps arrive at one or the other GW. If it will pose a problem, wifi space getting too crowded or something, then it's possible to separate the gateways more so they dont interfere.
Each sensor is supposed to be attached to a box to track it's individual temperature. The boxes can move from one room to another, hooking up to that rooms controller. A room can have everything from 1 box up to 20,000 boxes. Frequency of measurements will be about 15-30 minutes for a chart to be plotted on the web server. The boxes will never be more than 40 meters away from the controller.
Great, thanks for the quick reply.
64k * 126 sounds more than enough 
This is a Proof of Concept project I'm working on, and 20k is an extreme edge case, but still possible, so I have to take that many into account. Now to put theory into practice...
Hi!
Is it possible to have a controller collect sensor data from 20,000 sensors? What I want is thousands of arduino-sensors in a room, all taking some measurement of the temperature, and reporting to a hub/controller (arduino or rapsberry pi) which in turn relays that info using a GPS-shield to a web server. I'm hoping it can be done with some cheap RF/wifi transmitters. Using for example bluetooth can be pretty costly if multiplied by 20k.