The total supply of USD is $25 trillion.
First of all there is no ‘total supply’ of usd as it is not limited. There are also categories of which money we’re talking about. M0 is all the usd in its most liquid form: cash. M1 includes money that are used in exchanges such deposits and checking accounts. M2 includes M1 with the addition of savings and time deposits. M3 and M4 are considered broader categories of money that include large time frames and banks.
It’s difficult to determine how much usd really there is but currently there are only 2 Trillion USD in circulation.
Let's round it down to $21 trillion to make math easier. That would make, with a supply of 21 million BTC, that each coin is worth 1 million coin each.
Bitcoin’s value will not be determined by usd’s supply even if it was used to back up usd. Both fiat and crypto have fluctuating prices despite crypto being more volatile.