Polkadot does not have a maximum supply. DOT is inflationary. You can read more about that here: https://wiki.polkadot.network/docs/en/learn-staking/#inflation

There are currently just over 1 billion DOT in issuance. You can find more stats on https://polkadot.polkastats.io/ 


On the following page you can look up how much of the current supply is locked for staking: https://polkadot.js.org/apps/#/staking/targets


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


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