A UTXO is a spendable piece of Bitcoin. You could have a piece for 50BTC for example. But if you spend it for 10 BTC now you have two pieces, one for 10 and one for 40. Now you could make millions of pieces with 1BTC in theory.
Dust has been suspected to be an attempt at tracking. I don't know if that has been confirmed or if it has been successful, it was a long time since I heard about it, but that's the reason it's seen as bad.
Now this gives us an absolute upper bound for how many people own Bitcoin self-custodually.
Since there are many people that own 10 or more UTXOs, I'd estimate the max number of self custodial holders to be less than 10M, maybe less than 1M.