You gotta understand there were at least 10,000 of them before the heresy. Thats a lot of marines! and since only about 15-20% at best were sorcerers thats a lot of 2 wound marines running around to destroy. Also they aren't as blood thristy or confrontational as other legions so they should take many fewer losses.
I imagine the sorcerers can reseal souls into armor too so they may be able to repair some after battles and in 40k a model that is a casualty isn't always dead just unable to fight, so they might loss a leg during a battle and be counted as a casualty but after wards be fixed.
They don't recruit new rubric marines. Its not possable without casting another rubric which magnus forbid.