It depends on the standard of living that you consider satisfactory, and the country where you will live, since the cost of living is an important factor. But I believe that anywhere, 100 BTCs would be enough in twenty years.
For me 100
BTC is not going to be enough just to survive your remaining days here in world if you are going to retire in 20 years.
Because we are not sure what is going to be the value of 1 bitcoin by that time. I guess it is going to be a good range if you need 1,000
BTC and higher.
So whatever is going to be the value of bitcoin by that time it would suffice.