We don't publish any usage limits other than stating it's a free service that works on a best effort basis for reasonable usage, with the interpretation of those terms at our discretion. If you are not comfortable with that, you should set up your own TURN server.
It's hard for anyone to tell what the usage would be anyway, as it is only used when direct connections fail, and it's difficult to guess what proportion of devices will be resorting to TURN for any particular game audience.