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.