There are definitely more than there needs to be, but then you don't have to hire them. Look at it this way... Rome makes way too much money once you take Italy and mercenaries are expensive. Your generals are dimwits and morons as it is, even more-so if your treasury sites above 50,000 every turn, so use that money to hire mercenaries instead of recruiting allies from other cities.

I definitely think there needs to be less mercenaries though, or at least a much longer respawn rate on them, especially in Seleukid lands and lands close to it, or if not then stop giving them so much money every turn so that they can recruit a unit from every single city AND hire every single mercenary available every single turn... it's ridiculous trying to play as Armenia or Pontus... just when you think you're making progress they show up with a full stack of Misthophoroi Phalangitai and Thureophoroi, with a 10-star general leading them no less.