If bitcoin price grows 40% a year from now on, that will be 1000 times of present value in 20 years. The price is about $400 by the end of this year, so 1 bitcoin will be worth $0.4 million. If you have 10 bitcoin, you can have comfortable retirement life.
You are right. But I believe when more people adopt bitcoin the value of bitcoin will go exponentially. So I assume it may reach one million or 10 millions also. So probably only one bitcoin also would be more than enough. 10 bitcoins must be much safer side.