Do you want them now to breed, or just for show until later?
I would think two is more than enough to get quite a few of them. However, you might want to think this through before going ahead with your plan, since their corpses pollute the water horribly (hinting at not having too many ).
It's actually easy to control apple snail's numbers, because they lay their eggs above water, so you can remove as many as you need to. I'd say get two or three.
You'll need a group of say 5 to get males and females.
They will need to be about 1.5" - 2" to breed.
we have paired ours off now and the breeding pair have laid eggs twice now - this batch have only been hatched a few days and already there is a lot of snail poop on our sand
I have over 11 of them in a 30 with several other snails and fish but no eggs yet. It's funny how the people who don't want eggs get them yet when I do I'm not. It might just be they're not quite sexually fertile yet. They're very close if that's the case and I might get an explosion in a month or two. I have just one in a 10 myself at the moment but I'm contemplating adding some more to it and trying my luck there.