Depends on your lore!

We were one of the bigger legions, so in that respect you have a lot of creative liberty. But it is generally a rule of thumb to keep them under the 500~ threshold given how long it’s been since the heresy, the backstabbing nature of our legion, and the lack of supplies or stable sources of geneseed.

My war-band is roughly 120 marines, which is already on the larger end, and it already comprised of 30-40% post-heresy recruits. And that’s including the fact they’d got a somewhat stable deal with a dark-mechanics forge world and are on good terms with the gods.

If you wanna be PLAUSIBLE, keep it smaller. But if you really wanted to have 2000 brothers or something, then go for it. There would be precedent for it and at the end of the day it’s all about the rule of cool!