Lots, but I find the trick is to buy things that they would be getting anyway - so its not extra presents, just extra wrapping paper. So mine always get new vests, tights, socks, PJs, Dressing gown / slippers (if needed), pants, toothbrush, big tub of showergel, hair gel etc etc. None of which I can't not buy. Just I choose to save them until Christmas and wrap them up.
Then Santa does buy them little treats that they are not normally allowed - so chocolate cereal (huge tub), bubble gum etc etc.. (I like this one because it is what convinces DD1 (almost 8) that Santa must be real because she knows I hate bubble gum and would never allow her to have it
).
They also get some practical things, which whilst are not essential, are useful to have. So now they're getting glasses rather than melamine cups I tend to get a nice (£1) child friendly glass, cutlery, their own (funky) plates, name signs for bedroom, new crayons and colouring books etc...
Finally, they get a few toys. So it amounts to a huge big pile, but if you took out the things they would get anyway at other times of the year, then actually it would amount to no more than half a dozen presents. I just prefer not to throw items at my children all year round and instead they get a year's supply of pants at Christmas instead.