I would personally advise you to get around 50 just to be safe. Although, there is no way i see bitcoin being anywhere over 10,000$ anytime soon at all. If they go up to 3-5k$, well happy times for you!
50 is not enough actually, you need at least 100 BTC and in 20 years you can start saving now, regular saving is very important to accumulate a good number of bitcoin by 20 years and the amount or value will appreciate and that will be good for your investment, you will enjoy then.
100 BTC is the minimum amount that is very appropriate for the next 20 years. we do not know the price in the future, so you must be anticipated by having a minimal amount of decent, when in the future rising prices are not very high.