You can't over feed an iguana. Feed him as much as he will eat.
Generally speaking, though, ig food is usually measured in tablespoons and cups. You'll be hard pressed to find advice on how many pieces to give, since the amount will vary based on how small it is cut up.
Collard greens are a great staple green, but you really need to add more variety. Throw in at least 2 more greens every day, and add at least one more veggie. Try grated squash

Also, don't give fruit every day unless you're just adding a tiny amount of fruit to a huge amount of greens and veggies. The fruit fills them up, but is less nutritious by far. It's like letting a human child fill up on cake and candy and then maybe having a veggie or two for dessert... what is the liklihood of that happening?