Here's the Real Story . . .
There are 125 distinct cards in the Assassins release. Of those, only 123 are available in Booster Packs. For those of you keeping score at home, that leaves two cards unaccounted-for. Judge Crater has them, and he . . . [THWACK]
So, there are either 125 or 123 Assassins cards, depending on how you look at it.