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 actually depends on how much Bitcoin you want to retire with because not everybody is going to retire with the same amount of Bitcoin.
Some people want more Bitcoin and some people are already happy with a couple of them it depends on the person and what the value is.
If there are more than 10 million people retire with the same amount of bitcoin, you only need 2 bitcoin.