I have found that the best number for me and my house is 2. Doesn't matter what the 2 are, I've had 1 dog and 1 cat, and now have 2 cats. We've had numerous combinations in the past the most being 8 hamsters, 1 dog and 3 cats. 2 is the magic number around here. Works perfectly, I don't feel guilty leaving for work, and the vet bills are minimized.