There are roughly 35 million millionaires in the world. There will only ever be a maximum of 21 million Bitcoins. So there will never be enough Bitcoins for every millionaire to have one.
Thinking about the average millionaire, I would bet that at least 90% on gold in some shape or form. As Bitcoin becomes mainstream, I thinking at least 50% of millionaires will want one.
With a set up like this, Bitcoin could easily reach a price of 1 million dollars!
Reference article:
https://cointelegraph.com/news/bitcoin-price-might-exceed-1-million-more-millionaires-in-world-than-bitcoins