Lets say I want to buy bitcoins today and store them for 20 years to use a my retirement fund. Live comfortably, but don't need to buy a yacht or anything. How many do you think I would need to buy?
I think that that is going to depend on what the price is going to be in the future so at the moment it is not save to say what is going to be enough for the future.
You will have allot of time to earn Bitcoin in 20 years so you should get enough of them.
No one knows what can happen after 20years and saving or holding on to bitcoins for such a long time will secure your future. I suggest that you continue to earn bitcoins as much as possible as the day pass by and after 20years, I am pretty sure that you have saved enough for your retirement.