It's quiet high - in my latest save I have 16 "cows", 8 security, 8 office girls, 4 cooks, 4 waitresses, 8 "Spa" personal, around 10 maids, 5 test subjects for the aunty, 1 lab help, 4 librarians and lastly 4 bedroom maidens ... you do the math^^
I have also a lot of room for some more - still a couple empty rooms left.
Also 6*4 slaves inside the dungeon, chained up for max obedience - when they reach 100 I sell them for max profit ^^
Oh, do guests count too? I have 8 of my children running inside the mansion too ...
But one I can say for sure,
the lag is real by now ... therefore better not to go overboard like me with your house quests if you want a more "fluid" game experience