BTW, I read (with a big surprise) that there are only about 1 mil wallet addresses with at least one btc.
I thought that there were at least 2-5 times more.
There is a cost to making a wallet (the costs with keeping track of things, which can be minimal, but also the costs of fees) however it can be quite low, so even if you hodl tons of BTC, they can be spread on many wallets.
And then, you're talking about addresses not wallet, one wallet can have plenty of coins, all on different addresses.