The buckshot rules are based on 00 buckshot. Each pellet does 1d6 damage. So, if you see "2d6" for buckshot damage, 2d6 pellets hit targets, each of which do 1d6 damage. Shotguns using buckshot are to an extent area weapons; if your targets are at medium range, the number of pellets indicated may hit more than one target, and are treated as automatic weapons fire for this purpose. At short range, all the pellets hit one target (messy!). One my Shotgun Ammo page
http://www.pmulcahy.com/ammunition/shotgun_shells.htm
I have rules for 0 buckshot, different chokes, and IIRC, other shotgun ammo types.