We speak of Bitcoin's fixed supply being the maximum number of coins that can ever be created, which is 21 million.

Even though they are not all in circulation we still talk in terms of 21 million being the maximum amount of Bitcoin that is ever possible to exist.

You can refer to the coins that have already been mined as
"Circulating supply"

And you can refer to the maximum number of coins that can ever be mined as:
"Total supply"

Fiat has no limit on the Total Supply and no one really knows how much Circulating Supply there is.