Is there a number of TV's you can have activated on your ROKU account at any given time?
You mean a finite number. Im not sure. But you would run into problems with
Premium(paid for) Channels which I'm sure limit the amount of streams.
You can certainly run a few TV's on one account.
How many Roku devises are you allowed to have on your account?
There is no known limit to the number of devices an account can have. If you get someone telling you that you need to pay a fee of any kind, they are scammers and hang up on them. There is never a Roku fee to register a device or for customer support.