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?
Since a number of year has passed when you retire we don't know the price of the bitcoin, it can be so much higher than the price today or even much lower so just get as many as you can.
You should also have some retirement fund in your current job of course and your bitcoin should just be an add-ons to it