I currently don't know how much BTC I will need.
Once retired, then something will be rotten, so don't do it.
Perhaps get 21 BTC,at least?
That would depend on your lifestyle in life and your dream life after 20 years, if you want a very simple life then 21 BTC as your projection would be a good amount of money when you cash out after 20 years, so just need to hope and pray that price will be moving in increasing direction.