i think like 5 bitcoins will be enough, as you should have like 500k$ then
what? of course it wouldnt be enough and in my opinion bitcoin is not going to be grown enough, i would need like 30 bitcoins at least
I think we can not correctly predict the price of bitcoin will happen in the future or 20 years to come, but maybe you can estimate what price is going to happen and then you can formulate how much bitcoin that you need in the future ?
But still its very difficult to calculate the price after 20 years as you cannot come to any accurate value, so without thinking of the price I think we should at least have 200 btc for retirement.