1. The absolute core members are easy to figure out. You start with the Big Four of Cyclops, Phoenix, Wolverine, and Storm. Those four are literally in every incarnation of the X-Men, from comics, to cartoons, to movies. They were the four picked to be the team in the first movie, and also the four picked for the Mangaverse book. There's a reason it's always them. They, with Professor X, are always who first come to mind when you think "The X-Men".

The rest of the "core" X-Men can be figured out the same way: Who turns up in just about every incarnation. Who had made the comics, movies, and cartoons? You actually end up with a few surprises. After the above five, there's Rogue, Iceman, Nightcrawler, Shadowcat, Colossus, Beast, and, surprisingly, Jubilee.

2. Damn, that's too many to count :) Especially since the various X-Corp branches are considered X-Men.

3. See above :) It's also hard to say who's "official" current X-Men, given Storm's bunch still calling themselves that.

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