Even only one bitcoin is more than enough after 20 years. But by considering 2036 commodity prices, you may need 10 Bitcoins to lead your rest of life happier.
Why would it be enough? How would you know that BTC will be worth many, many thousands USD per?
Nobody knows where BTC will be in one year, why would it be possible to know what it will be worth in 20? Bitcoin will bring about a revolution but that doesn't mean that Bitcoin won't itself die in the progress. No revolution without its toll in blood. Remember?