No.  The supply stays the same.  A fork is introducing a new client and new coin.

An accurate way to phrase the limit is that the current main chain will never have more than 21 million coins.  While a new fork may become the main chain, anyone who wishes to preserve the 21 million limit can continue using the chain with the previous (and arguably correct) supply.
Usability is about 5tps in good conditions (with a large number of people using SegWit transactions).