Well you will need 3 to help fight with you.
You need 1 cook.
You need 1 in the Lab.
You need at the start 1 to clean but the more people you get you will soon need at least 1 more if not 2 for a total of 3 when the house is full.
You need several to hunt and gather to supply food. (How many depends on how many people you have, the more people the more food they eat.)
I also recommend you get a "Headgirl" which I use the sister for and set it on "Patronage" this will help keep the house from blowing up in fights between the girls. (You need to make the girl as close to 100 on looks as possible and give or raise her charm up as high as you can get it.)
To see if a girl is fighting with or being a lover to any other girl you look at the "Statistics" and in the right hand side show who she knows and what kind of relationship she is having with each.
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(((If they are fighting then put them in the same job for several weeks and they will work it out.)))
I use my fighting girls for hunting so all in all if you want all the Main Npc's to go along with what you need then you are looking at about 24, I use the main NPC's to bring in the money with jobs with most of them working at the Mage Guild.
Both Emily and Tisha I use for cleaning along with Cail plus after they all become friends they sleep in my MC bed keeping him warm at nights.