We support up to 12 controllers in a mobility domain. You will want to make sure you are careful when setting up large mobility domains. If a user starts out in subnet x and roams to a controller with the same SSID that is in subnet y, all the broadcast traffic for x and y now have to be sent by the ap's radio. So if the broadcast domains are large to start, you could be doubling the broadcasts a radio has to send.
I know that 12 controllers with 5000 ap's was the last number I had seen tested against NetSight. If you require more testing, I would suggest you work with your Extreme Sales Rep to help get a deployment of that size confirmed by our Product Management Team.