Polkadot does not have a maximum supply. DOT is inflationary. You can read more about that here.

There are currently just over 1 billion DOT in issuance. You can find more stats on Polkastats

On the Polkadot-JS UI you can look up how much of the current supply is locked for staking.

You can find the information on the top right of the page, as per this example: