20,999,999 Bitcoin, because it's impossible you would still lack of money when every people in this world can only have 1 Bitcoin as long as you didn't sold your coins.

If you think it looks impossible, then you need to know how much money you think is enough for you to retire and how much money you can earn without you need to work. It's not about retiring in Bitcoin, but it's about how much you can earn when you're sleep.