More rodents smell more.
Less time for play with each rodent.
More time spent for basic needs.
Bigger cost.
More rodents mean you might need vet care.
Breeding rodents means you have to find good homes for all the babies.
You can't always find good homes.
You won't make money out of breeding mice if you do it responsibly.
If your only breeding to see what you will get please don't.