most of the time male mice WILL fight to the death. you can get them neutered, however.
some people will say you can keep males together if they grew up together.. no, sorry but once they hit a certain age they will fight (and fight to the death in MANY cases). I've only had two male mice get along together, and one was hand raised by me as a baby, while the other was a frizzle from a breeder.
you really shouldn't breed unless you have good homes lined up, and they come from a nice lineage from respectable breeders... otherwise you could wind up with MANY babies that have no homes, or wind up being fed to something inhumanely.... or just not cared for. then each male will eventually need to be kept seperate from the other males.... I took in a mess of mice once, a girl bought two mice (one male, one female).. they had babies. then had babies again after the first litter was weaned. I wound up taking them all in, and NONE found homes. they were all PEW, and I wound up with a 20 gallon long tank for the girls, and the boys were in a 55 gallon tank. they STILL fought despite many food bowls, water bowls, different wheels, hiding spots, etc. they were OK for a few months, but eventually they just started tearing at each other and I had to get each one their own tank... try having about 7 tanks, with one mouse in each.. pain to clean. at least there were more girls than boys born......
2 rats, 2 mini rex bunnies, 2 cats, 1 prairie dog, 4 species breeding millipedes, 23 various different species tarantulas, 2 Heteropoda davidbowie huntsman spiders, 3 Liocheles australasiae scorps, 20+ Hogna helluo wolf spiderlings, etc.
spiders are addictive... and super easy to care for!