Mice are colony animals at heart, but group living only works when sex, numbers, and space are all right at once. The short answer is that females belong in a group, males usually do not, and the cage has to grow with the number of mice sleeping in it.
Females Together: Start With Three
A colony of at least three females is the common working pattern, and the reasoning is practical rather than sentimental. Mice groom each other, sleep in a shared pile, and settle better with company, and in a trio the death of one mouse still leaves a bonded pair instead of a lone animal grieving in an empty cage. Larger female groups can work well too, provided the housing grows with them and introductions are handled carefully.
Introduce new females on neutral ground, watch the first days closely, and expect some squeaking and chasing while a ranking settles. Persistent wounding, one mouse being kept away from food and water, or blood drawn means the pairing is not working and the mice need separating and a rethink, not more time to fight it out. Keep spare housing ready before you need it, because a group that must split tonight cannot wait for a delivery.
Space, Bars, Bedding, and Hides
Space decides whether a group stays peaceful. About a 20-gallon-long tank is a starting footprint for a small group, and more floor space is always welcome, because mice use ground area for feeding, digging, and getting away from each other. If any part of the housing is barred, spacing around a quarter inch keeps small mice from slipping through or getting stuck halfway, a risk owners underestimate because a mouse's body compresses more than it looks like it should.
Fill the floor with deep bedding for burrowing, add a solid wheel, tunnels, and at least as many hides as there are mice, with extra exits so no mouse can be cornered inside a single-door house. Provide more than one water point and scatter food or use more than one bowl in a larger group, so a confident mouse cannot guard every resource at once. Clean in sections rather than stripping every scent at once, because a completely scent-free cage can restart ranking disputes in an otherwise settled group.
Males: Singly or Apart, Plainly
Intact male mice fight, and their fights injure. Brothers raised together may coexist for a while and still turn on each other as they mature, so the plain guidance is to keep intact males singly or in separate housing, each with his own wheel, hides, and enrichment to make a solo life a full one. A single male needs more human interaction and cage interest, not less care because he lives alone.
Count during cleaning and floor time, not only in the cage. Mice are small, fast, and inventive about hiding inside a sleeve, a tunnel, or a pile of bedding headed for the trash, so a headcount out and a headcount back in belongs in every handling session, and play areas need the same escape check as the cage itself: gaps under doors blocked, other pets out, and no chair legs hiding a mouse from view. A group of three or four mice multiplies that quiet risk, and the keeper who counts aloud, every time, is the keeper who never discovers a missing mouse an hour later by its absence from the food bowl.
Whatever the group, keep a household headcount and care list that names the mice, the feeding and water checks, and who cleaned which section, because a cage of similar-looking mice invites double feeding and missed checks when several people help. That shared list is where KinsLe helps most, since the Family Sharing view shows every helper the current headcount and last water change at a glance.