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.
I think it's a bit premature for that, in terms of purchasing power. When BTC has large enough purchasing power to actually compare to a major currency, we can talk. But the economy is still tiny for now.