Of the many, many crypto address out there, how many are really being used?
One way to know, would be to check 'active' wallets, the ones that have a recent transaction - you define the meaning of recent (1year? 2years? more?).

The problem is that old wallets may still be owned (people just hodling) and accessible, and bunch of wallets are also owned by the same people.

You can use this site : https://bitinfocharts.com/bitcoin/explorer/ to check some interesting data about existing wallets.