Mice can reproduce fast and furiously more than you can imagine. In proper conditions, they can have six to seven babies in a litter as quickly as every 21 days or so. So you may not realize (or want to think about!) how many of them are in your house, but you can be almost certain there are more than one. To stop an invasion of mice, you need more than a few mouse traps to eliminate the problem quickly. The most effective strategy is to place one mouse trap every 2 to 3 feet along the wall where you've seen signs of activity. In the highest-trafficked areas, set mouse traps in pairs as close as an inch apart.