The term millionaire is rooted in fiat. Should BTC replace fiat, 10 coins should be enough to classify as wealthy. That number can be argued. But we need a new term for someone with enough BTC that they are wealthy.
Should BTC replace fiat entirely, 1 coin should be enough to classify as wealthy.
Should BTC "only" reach the market cap of gold, 3 coins should still be enough to classify as wealthy.
Assuming wealthy = $1,000,000 in 2014 or top 7% of American society or top 1% of the world.
One day I hope to become a bitcoinaire.