Sorry for clickbaity title, obviously there will only be 21M BTC.
But with more banks, institutions, and CEXs trying to custody the BTC and creating ETFs, options, and perhaps other derivatives in the future, how can we be sure there won't be an "infinite" supply of these synthetic "shares" of BTC?
The amount of bitcoins we can buy is a fixed amount. If other platforms decide to sell "bitcoins" but it is not actually backed by bitcoin, is that what your question is? Well thankfully you can see transactions in the blockchain, right? You should always try to check for this. Even then I do not think you should be buying bitcoins from ETFs or other institutions.