There are no guarantees in life. And if you are expecting someone else to "take care of you", then you have no business procreating.
Back to the original question, 1 would do it if you raise him/her right. I suppose the odds increase as you go up in number. If you raise them poorly, then 10 might not be enough. But if being taken care of is your concern, then I also assume that $ is a concern for you as well, and kids ain't cheap, so the more you have the more it costs. Don't have any and you can more likely afford to pay someone to take care of you.