I dont remember, how big is Shadow Garden currently is in this side story?
They just founded it. So it's just the core girls and maybe a couple dozen or so rescues? It's more than just the Shadows if they're building Mitsugoshi, but they certainly don't have the hundreds of girls that they've got by the time the main story really starts.
It's a little unclear how quickly it's actually growing. This chapter just spelled out that right now only Cid, Alpha, and Epsilon can fix the Possessed, so recruiting
should be slow.
Unless they rescued a whole gaggle of Possessed at once.