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?
It id all going to depend on what the value of the Bitcoin at that moment is because that will decide how much money you are going to make.
So it is hard to say on how many Bitcoin you are going to need for the future, just save enough.