I do most of my pheasant hunting these days with my 16''s. But back when I owned a pair of Army&Navy 2 1/2" 12's, (made by Webley & Scott) I shot a lot of Kent Gamebore Pure Gold 30 Gram Brit #6shot. Going by "the book" , that load should contain 181 pellets. A favorite 12ga pheasant load of late American shotgun Gurus Michael McIntosh, Gene Hill, DonZutz, and Bob Brister was 1 1/4 oz of US6 shot which is just slightly larger than British 6 at 187 pellets in that load. One cold winter day, with nothing better to do, I literally dug into those two loads and discovered that the pellet counts were close enough to the "book" counts that they are very close to being ballistic twins. And both of them seemed to be reliable pheasant killers. But on foreign brands of shotgun ammo, it's always good to ask whether the shot size is US or foreign.
We also measure velocity differently than do our counterparts on the other side of the Atlantic, but that's an entirely different and even more complicated subject. Somewhere along the line, assuming Eley still puts it out, they used to publish a very handy little pocket book called " Eley shooter's diary" which helps to break the code on shotshells and ballistics. If you ever see one of those, make sure you pick one up. It is a small treasure of shotshell facts.