I think you are explaining very well.
The user first chooses type of letter and has 4 choices (say type A,B,C,D)
then
The user first chooses how many sheets and has 10 sheets available (sheet 1,2,3...10)

My questions
Can the user choose as many sheets as he wants (max 10)?
If he chooses 5 sheets which sheets are they? Is that dependant on the type of letter, or is the user free to decide?

thanks