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.
Amout is based on country you live
It may be that 1000 wont be enough and soe where 10 is enough
10 BTC is impossible to be enough to live off even half a life. Remember, you need to have food, water, supplies, necessary things like toothpaste, toilet paper etc, electronics, you have to pay your rent/facilities and many others. With $6k you can basically do nothing, you can spend them off in maximum two months. Or you can go and live like a homeless, but that won't help you with anything because it's not the style of living you really want.