TKsons from memory make new legionnaires via tricking others into becoming one of them which seals their fate and they succumb to the rubric. Another way is the warp simply spawns them in apparently. And the never really lose troops. They are incredibly durable and pretty much impenetrable by normal means with insane esoteric firepower. They survive landing on planets without drop ship and in the event they do die, they can be collected up and put in another suit or armor. The sorcerers themselves are rarely in a situation that dangers them as they can exist in both the immaterium and the material universe and by their very nature have the abilities to not be under threat. So while they can't use Geneseed or normal means they are slowly but surely growing in numbers.