Yes, you are right. And there are currently more than double that - 36M millionaires:
http://www.businessinsider.com/worlds-millionaires-own-nearly-half-the-total-wealth-2017-11
Even if only half of them wanted to own a single bitcoin, there wouldn't be enough.
So you mean bitcoin millionaires?
How many of those have enough to put $1million in to bitcoin just to say have a bitcoin. If you had $1million would you buy a house or a bitcoin.
What about the rest of the disposable income and other investments, just because they have a million dollars doesn't mean they have a million dollars they can do what ever they want with.
And then what about other coins, BTC has only been taking about 50% of cryptomarket share. Do you think they wouldn't split across other coins if they were to put into crypto?
Yes, you are right. And there are currently more than double that - 36M millionaires:
http://www.businessinsider.com/worlds-millionaires-own-nearly-half-the-total-wealth-2017-11
Even if only half of them wanted to own a single bitcoin, there wouldn't be enough.