Lettuce does not make turtles fat, pellets do! Make sure you read the nutrition pages litefoot linked to! Then you can check in again
Also, the number of pellets only matters in relation to how big the pellets are. We might not know how big the pellets are for the particular brand of pellets you are feeding. You need to compare with head size. For example, I feed 12 pellets every other day to my 4 inch turtle, but they are fairly small pellets. Someone else might feed 2 or 5 if they are using bigger pellets.