[@Savage] Actually, it turns out most of the Legions in canon had substantially more than 10000 members for most of their run, and only a few had less than that; the highest was the Ultramarines, with a frankly staggering 250000 members, whilst some seven or eight other Legions had more than 100000 members at the time the Horus Heresy began. With that in mind, I'm not going to put any direct limit on how high a Legion's strength can be, though I recommend you keep it sensible; a million Astartes isn't going to happen, no matter how hard you beg for it.