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..
Yeah depending only on bitcoins would be an risky business, as we are not sure what would be the value of bitcoins in future, its better if we invest in multiple products so that it can take care of our retirement.