...is there supposed to be...
Well, there isn't really any "supposed". Makes it sound like there is a law or something.
It's whatever you want, whatever you need, whatever you like.
Many people believe strongly in single-set training. That doesn't sound good to me, but I've never tried it. Some people swear by it. Most people do some form of multiple-set training, but the more sets you add the less difference it makes. You don't even have to do it the same way for every lift. You can do your "main" lift single-set and do your assistance lifts multiple sets. You could do your main lift 3 sets or 5 sets, then do several assistance lifts single-set.
What are your training goals? Strength? Size? Endurance? Fun? Prep for a particular sport?