Off course theoretically there can never be more than about 500 000 people with 50 BTC but real number of course will be much more lower because there are huge whales with lot more and all the lost coins (and unmined coins at the moment.)
rpietila has a thread dedicated to trying to answer these questions. Most evidence suggests that bitcoin wealth can be approximated using a Pareto distribution [e.g., the top 20% hold 80% of the wealth, and the top 20% of the top 20% hold 80% of the 80%...all the way up to Satoshi]. Of course, the exact distribution can never be know. Please take a look here:
https://bitcointalksearch.org/topic/distribution-of-bitcoin-wealth-by-owner-316297
