I thought there were plenty of Aeons. I certainly wouldn't object to there being more of them, but I'm not disappointed with the number given to us.

I actually thought that the number was perfect. Any more Aeons, and you start getting lost in how many are available. In games where the summon creatures do an attack and then disappear, then a higher number is fine.
However, when the summon creatures become controllable fighters, limiting the number is ideal in my opinion, or else, as Ultima said, you start relying more on them than on your party members.
As it is, if you take the time to charge up all of their Overdrives, by the time you obtain Bahamut, you can pretty much breeze through the rest of the game by summoning one after the other. The various Seymour forms can be taken care of very easily by doing this (as well as Spectral Keeper, and Yunalesca, and Sin, and Braska's Final Aeon...soooo easy...)

I think it would have been better if the aeons had more abilities already learned but I thought the number was fine.