Just to get an idea of how many people are using bitcoin.
It is impossible to know completely. But there are some websites and they calculate some statistics. And they know how many wallet has a BTC. But someone can have more than one wallet. And no one knows real number who has how many wallet.
For example:
https://www.blockchain.com/charts/n-unique-addressesHow much the wallet is could be possibly counted, but how much people that own the wallet that have btc, we are never know. It impossible the detailed information can be found. Just wasting life time in digging it.