Lets say I want to buy bitcoins today and store them for 20 years to use a my retirement fund. Live comfortably, but don't need to buy a yacht or anything. How many do you think I would need to buy?
I think the more you buy bitcoins, the better. But since you said , not so much money is required as to buy yachts aand mansions buying just three to four bitcoins is enough since in 20 years bitcoins' value will shoot up to the sky. The chances are of you becoming a millionaire with just four to five , dude! If you have financial stability now, I think you should go for it.
I would also advise you to read more about bitcoin, learn more about how things can go wrong and how positive things can be, analyze if you are ready for such a commitment and will you hold or not. When you completely believe in your investment, invest only the amount which you can afford to lose and not have a direct impact on your routine lifestyle.