I just have one child and no set rules. I buy a lot of things secondhand which can mean a bundle of toys that I count as one gift when in reality it might actually be 6 or 7 different toys. For example, last year he wanted a Batcave and a Rescue Bots playset, which was 2 presents. But I bought the Batcave with a selection of other heroes, villains and vehicles. And the Rescue Bot base with all the Bots, some character figures and villains. Both bundles were less than the price of the two toys he wanted new.
On Christmas eve, I set up the two playsets with the figures they originally came with and a few extras, like a Batmobile, Batgirl and the Joker and an extra Bot with the Rescue Bot set. Then all the extras go in the stocking which is opened last. I also usually give him a set or two that he hasn't asked for.
DS usually asks for 2 things and I give him 3 or 4, though if I was buying new, it would actually be 20ish toys. His stocking is made up of extras from the bundles, toys I make myself, special books, dress up outfit and maybe a special teddy or cushion. And a few edible treats of fruits and chocolate.
If possible I order something from the US or Australia that was never released here or a retro toy from the 80s/90s that he has seen on Youtube and loves. I also make custom toys inspired by customs he's seen on youtube or a fantasy toy he has imagined (this year I'm making Monster 500 cars with the cast of 5 Nights at Freddies in place of the monsters). It adds to the magic of Santa as I have told him that it's just not at all possible to get those toys delivered here/anymore/in existence yet Santa and his elves have managed to come up trumps.