The total supply of DOT changes every era. DOT is inflationary, with a fixed annual inflation of 10%, and doesn't have a maximum supply. This fixed inflation can be changed through a referendum.

New DOT are created every era. A portion of that newly created tokens is used to reward validators and nominators for securing the network, and the rest are sent to the Polkadot Treasury.

How much goes to staking rewards and how much goes to treasury depends on the percentage of the total supply of DOT that is staked compared to the ideal percentage of staked DOT. When these two are equal, the full 10% goes towards staking rewards. You can read more about the inflation model in this Wiki article.

In the following example, the ideal staked percentage is 75%, and the current staked percentage is 47.5%, which gives an inflation of 7.3%. This means that out of the 10% annual inflation, 7.3% goes towards staking rewards, and the rest 2.7% goes to the treasury. You can check the current numbers on Polkadot-JS UI under Network > Staking > Overview.

What is the current supply of DOT?

Most Polkadot block explorers display the circulating supply of DOT, and they provide an easy way to view that metric, usually on their homepage.

Alternatively, you can query the chain state directly to get that information. You can do that on Polkadot-JS UI by navigating to Developer > Chain state and querying `balances.totalIssuance()`. This number is expressed in Plancks (the smallest subdivision of DOT), so it needs to be divided by 1010 to get the amount in DOT.