How many more monster girls will fit into the harem, anyway? And I mean that physically: while he has a big house for a single guy his age, it's getting pretty damn cramped in there. Especially when most of the girls take up more space than a human would (it's a miracle that Centaurea doesn't constantly hit her head on doorways, lamps and such).
Also, with each addition to the harem, that guy's home will get renovated to accommodate the needs of the new girl. Arachnera was a subversion (nor sure yet) since she didn't get limited by 2-dimentional locomotion, e.g. she can sleep on the ceiling.
I seriously hope he stops adding to the harem, at least for a while. (If he absolutely to add more characters, it's better if they get thrown into MON, instead.) The characters that are there right now have basically only had one chapter to introduce them, and then just one more chapter to actually flesh out their character a little. (Lorelei's total characterization thus far has essentially only amounted to "tragedy maniac".)
The series needs more time spent developing the characters it already has than more paper-thin characters.
I agree with NWSiaCB, I hope it will take a while before another harem member is introduced. It's fine if it's just new "support" monster girls like the kobold gym instructor, though.