I think with the inflation of BTC a 100 btc is a must you need to earn that much to have your life free of financial problems, but supposedly a 10 btc is great though because BTC is getting higher and higher overtime.
that should be good amount of coin to have in 20 years
for price we can only speculate and guess what will it be will be like it is now (which i dont think so) or it will be much higher (some even speculate over 10k)
for any amount you as user need to decide what amount you need in 20 year 200k or 1mill $ in btc