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.
even 100 bitcoins wouldnt be enough in my opinion you would need like a thousand or something like that