A max supply is a rule in code. Bitcoin's 21m is the well-known example, enforced by a halving schedule that reduces the block-reward until issuance ends.
Caps are not automatically credible. If a contract retains a mint function controlled by an upgradeable admin, the stated cap is a promise, not a constraint. Verify on-chain rather than trusting a website.
Also note that "capped" says nothing about value. Scarcity without demand is just a small illiquid market, and plenty of capped tokens have fallen 95% or more while the cap remained perfectly intact.
Related: total-supply, block-reward, halving, admin-key-risk