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.
Indeed. It also depends on the country you want to retire and live and spend your retirement fund. I would suggest to live here in our country since cost of living here is cheaper just like countries in Asia. I would make more bitcoin as far and as long as I can because I do not know what its price is for the next 20 years.
If you want to retire in a country with cheap living cost, it is possible to have 100 bitcoins in a few decades.