If you are thinking to retire in 20 years based on fiat, then it is not assumable.
But if you want to retire based on just Bitcoins, and if it ever gets completely used-to by the whole world, then even 100 Bitcoins would do.
If the whole world should use Bitcoin one day then you will need much less than 100 BTC.
The whole world does not only mean the average people using it, but also part of the financial markets, foreign remittances, global gambling,e-commerce, new unkown industries and ways of generating income in form of crypto currencies, etc.
This would be a scenario where we would talk about bits to have a comfortable life.And owning 10 BTC for instance would make you rich.