To be safe, get two Bitcoins.
lol i agree with this. in 20 years 2 bitcoin may be worth 2,000,000$ and to me a 2m retirement fund would last if you plan on retiring at 60 and living untill 75 or whatever. specially if you have another income coming in still from ssi or whatever it may be
safe side get 5
I really doubt that Bitcoin will reach anywhere near $2,000,000 in just 20 years. Bitcoin's price may reach that amount, but not in 20 years, maybe like in 40 over years.