I think for being sure and rich you should at least get 100 bitcoins, otherwise, it would be a risky investment iff you buy 100 now and bitcoin would get shut down by governments and youre bitcoin would get useless
I think no one knows for sure how bitcoin that you need in the future. because no one knows what the price of bitcoin will happen in the future.
so I think 100 BTC may be sufficient, may not be enough for risk minimize I think you have to gather more bitcoin