It's 5 items for AF+1 to upgrade, and 10 for normal AF to upgrade. It's confusing though because the NPC assumes you have the +1 and talks about that first, then tells you about the normal version as an afterthought.

I think this system looks great. Abyssea seals were a huge hit, and this is basically the same thing, but without staggering and with guaranteed drops for each player.