All of your first aids are automatically played when you go to 0 or below, so the humiliate would stop all of them. You can't just play one of the first aids, wait for it to be humiliated, then use the others.
I don't think you MUST play all first aids -- heck, you can choose to die and end the game -- but any that you play in response to going to/below zero must be played at once. (So you can't play the one, see if you get humiliated, then play more.) But, hmm, I haven't thought about it much. I guess one may as well play them all...if you're Humiliated and the First Aid doesn't take effect, you're dead either way, whether you missed out on recovering 2 points or 6 or 10...
Well, yeah...you don't have to play them. After all, the rules say you can, not you must. Personally, I always play all the ones in my hand, but I suppose it could be an option not to. I guess you could have some kind of odd strategy where you hold on to some of them for just in case, but I figure it's best to use them all and draw that many more cards.
If you have more First Aids, I suppose there could be an advantage in (a) keeping your points low so as not to be too attractive a target to the next player around the circle, and (b) having those First Aids to bring you back if someone thinks they can off you with a Pimp Slap or something. But I don't generally think that deeply -- I'd drop 'em all, get all the points, and hope that I also redraw some defensive cards.