How much should kittens poop?

First off, thanks to all who have answered my kitten questions over the past week! I am fostering 2 orphaned kittens (from different litters) who are about 6 weeks old (my FIRST fosters which is why I'm so worried). Now for my next question:

How much should the kittens be pooping? These little guys were prematurely weaned from the bottle and put on solid food/kitten milk in a dish at 4 weeks (a little too early imo, but I didn't have custody at the time).

Anyway, they are eating well but they're pooping about 4 times per day each (or I should say, there are about 8 poops in the litter box each day.....not sure which poops are coming from which kitten). Is this many poops normal for 2 little kittens? It seems like a lot to me, but again this is my first experience with orphaned kittens so I don't have any basis of comparison. I should also add that I have dry kitten food out for them all the time and give them wet kitten food twice daily. I stopped giving them the kitten milk because it gave them diarrhea but they have fresh water all the time.

So the question is: 8 poops daily for 2 6-week old kittens-normal or not??

I have 4 six-week old fosters and I would guess they each poop about twice a day (between them and their mom the eating machine I just seem to be scooping it constantly ). I would go more by the poop itself and how the kittens are--is it solid and not runny and are they acting and eating normal? If so then I wouldn't worry about it. Also how much is there at a time, not to get too graphic but sometimes in one poop there's a fair bit there--so maybe they're doing the same amount as mine but just in smaller doses