Here the question is looking so simple but is not .
Because here OP is asking how much bitcoin needed to be millionaire in the future without mention the risk associated with bitcoin , I mean bitcoin's price is included with  much fluctuations So here chances are also for the down of price .
But if we are discussing about the only increase in the price only then I will say we should have at least 50-100 BTC because , if we have very low amount of bitcoin then after increase of bitcoin price still will have profit of some dollers only , So here having much amount of bitcoin may be result into the big profit which is best looking profit for everyone .