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I've been feeding him about 20 pellets a day and a few shrimp. But someone told me the shrimp has no nutritionally value.
Am I feeding him too many pellets a day? Also, what else can I add into his diet.

My turtle is about a year and half - two years old
picture of Squirtle:

That sounds like a lot of food for a 2 year old turtle. You should only be feeding enough to fit into his head if it were hollow and you should also work in fresh leafy greens.
Shrimp is good for an occasional treat but should not be part of their regular diet, it's too high in protein.

This link has some great info: http://www.redearslider.com/index_nutrition.html

He just seems like he is always hungry. So how many pellets for the day? 10?

Red eared turtles are big beggers, no matter how much you feed them they are always going to beg. As devilduck said, you should only be feeding enough to fit into his head if it were hollow. Otherwise he will turn fat.

I would bring it down even more. Use a pill box to resist offering too much.