Here's some good info on the subject:
http://www.redearslider.com/pellets.html
While you're at it, why don't you read all the info on nuitrition. it's very helpful.
You can't really say the number of pellets to feed because some pellets are little and some are big. So, as it says in the info cited above, you're supposed to feed the amount that would fit into the turtle's head if it were hollow every day while they are little.
After about a year, you decrease the amount as follows: give the turtle the amount that would fit in his/her head every other day (instead of every day.)