I think they are more rare then marines. They say the lost of 6 assassins will take an extremly long time to make up for.
Only in Death are we innocent!
It could also be that no one wants to talk about the assassins for fear of bringing their attention on themselves. It is extremely possible that those who talk about assassins "disappear", possibly along with their entire families. That would be enough incentive for most people to keep quiet. And those who don't keep quiet, well, they aren't around to talk for long.
92 Golf: Death From Within
Any plan where you lose your hat is a bad plan.
I thought the fluff was that the Squat homeworlds and colonies were et by Tyranids, leaving only scattered and embittered survivors around the place? That makes more sense. The survivirs would be less keen on risking their lives in battle and more keen on breeding like rabbits on some isolated world somwhere to try and rebuild their civilization.
As to assassins, it does make more sense for them to be kept mysterious and un-written about. in my opinion at least. They are covert agents, not an army like Adepta Sororitas.
Ask not the EldarGal a question, for she will give you three answers, all of which are puns and terrifying to know. Back off man, I'm a feminist. Ia! Ia! Gloppal Snode!