My dogs all have 3. 1 each for the harnesses and collars I trade back and forth on them wearing daily (1 less tag to move over, I figure) and one for their night collars (just extra collars with ID, reflective designs on them, and and little flashing lights attached). I just keep the night collars by the door to put on when we go for walks after dark. I just replaced them all because my phone number changed. The only info I have on them now is 2 phone numbers, so someone could get ahold of us if they were ever lost and found.

My cats tend to have just one each, but they don't really go outdoors, so I don't worry about them getting lost as much.