Tina,
I'm assuming that we are talking about the PRC exercise set.
Each Scott number is for a block of 4. Each stamp in the block is different (even though 2 look the same) because a different set ID number has been printed at the bottom of each stamp.
The entire set consists of 10 blocks of 4 (or 40 stamps total). All 40 stamps are different. They are usually collected in blocks (whether originals or reprints). Ironically, the only singles I have are used singles that went through genuine postal use.

k