General rule of thumb.. if you can label it, it makes diagnoses easier later on! 1 thing per relay?? If we're talking a electric fan or fuel pump, then yes. I wouldn't want a mixed load. And that's what you need to be watchful for. The total load of a circuit if you're going to apply multiple loads on a circuit. The wire has a rating and the relays will also be rated. You need to know the loading that you want to apply and don't overload / exceed the rating.