Vintage cards

Hi, I'm thinking about collecting vintage carded figures but before I do, I need to know what is meant by 12 back, 20 back etc.

Could anyone enlighten me?

Thanks in advance

Those numbers refer to the figures listed on the back, as in the "collect them all" type lists.
12 Back - the first 12 figures released (Han, Luke, Leia, Vader, R2, etc.)
20 Back the first 12, plus the next 8 (Greedo, R5, Hammerhead, etc.)
21 Back - the first 12, plus the next 8, and Boba Fett.
The numbers increase as did the number of figures released, for ESB and ROJ cards.

Thanks a lot Bel-Cam Jos, I feel i bit more clued up now!