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?
This question is very tricky because we don't know or don't have the knowledge to predict after 20 years what will happen with the internet neither with the bitcoin.
I think that you should count a million dollar equivalent in bitcoin, so around 2,000
BTC or more I think should be enough.
If you are a bitcoiner then there is no age limit for retirement, you can earn bitcoin till you are alive, so if you get retire from full time job then you can get full time in bitcoins after that.