Price is set by people, but supply is set by rules, and on-chain it can be counted to the last unit. Supply is the denominator of every valuation indicator in this course, so it pays to know exactly what's being counted.
Bitcoin's schedule
New bitcoin appears only as the block reward, which halves every 210,000 blocks, roughly every four years. The limit approaches 21 million. Issuance is known in advance, and the only uncertainty is how fast the blocks arrive.
Ethereum's two taps
Ethereum pays new ETH to validators and destroys the base fee of every transaction. Supply can rise or fall depending on which is bigger, so it has to be measured rather than scheduled.
Coins that will never move
Some coins are gone for good: lost keys, burned tokens, coins sent to unspendable addresses. They can't be identified with certainty, so "circulating supply" is always an upper bound on what can actually trade. Long-dormant coins that suddenly wake up are the reason the bound is left where it is.