Its hard to predict but you should need a lot of them to succeed with this also the question is if the bitcoin will be worth much in that year.
If so that you do not have a lot but from now on its still unpredictable and you would need a lot.
So the best is still to prepare for the worst scenario after 20 years, like the price drops very low.. so it would be good to collect as much as we can, but it should only be what you can afford to lose..better yet, invest your retirement in fiat and make bitcoin as a reserve..