Gosh I agonize over this!
Mine are little, 4 and 2, and I constantly change my mind about what I feel is right for them.
I don't have many people who buy for them. It's only really my mum and sister who buy for them which is absolutely fine of course, but I do take it into account obviously.
My mum is great though, she is generous and will ask me what I would like to get them so I can essentially make them a list of stuff I actually want them to have and then split part of the cost. Which works much better than having to make sure I have bought everything on a wish list and then getting random stuff and potential duplicates which usually are wasted.
I decided, despite reading about how it's good for kids to not have too much stuff especially when they are little, that the 4 present rule was just a bit too stingy for me.
So each child has 5 toy presents from us, about 4 non toy presents including pajamas and books, and about 4 good and thoughtful presents from my mum. They will then get a present off my sister and possibly my brother that's another 2, and then a stocking from Father Christmas and potentially 1 main present from father christmas. I'm literally still deciding how to do the while santa thing. It may be that we do one main toy from mum and dad and then all other toys we buy are from Santa.
So all in all including all relatives that will give them about 15 presents to open each, plus a stocking. I think I would be happy dropping that down to 10, but I don't think I could go lower than that and feel happy.
Sorry I am thinking out loud a bit really 