Never feel bad about asking a question. We all have things we are ignorant of. Ignorant just means we don't know yet.
There is no way I am sticking my hand in that box!
Their pecking can hurt, but if you wear heavy gloves, say those leather work gloves, you can check. When I was a boy on the farm, one of my chores was to gather the eggs around 5:00 p.m. If we had a broody, I'd pick her up, toss her out the hen house door, and check for fresh eggs. She would usually use the opportunity for food, water, and a poo. Some of them did not like it and I did not have heavy gloves. It was just something I had to do.
In general, what percentage of eggs usually hatch?
I can't answer it varies so much.
How many eggs would be too much for a Buff Orphington to cover?
They are big birds. I'd think they can handle several more than that but if you stick with 12, you'll be very safe.
It's too late now if there are too many, as I didn't get to mark them first. The other day, she seemed to start to go broody, but then went onto the perch with the others that night. There were 6 eggs then. It had only been from that day. Should I have just left them?
I suggest collecting the eggs every day, then when one goes broody, mark the eggs you want her to hatch and put them under her after you are sure she is broody. That way they all start at the same time. Put golf balls or plastic eggs in the nest for a couple of days and see if she will stay on them. Even better, if your set-up allows, is to set up a broody pen for her, where she is locked in with the fake eggs away from the other hens, but has enough room for her own water, food, and room to get off the nest and go poo. Then after you see she is truly broody, you can give her the real eggs. As it is now, I'd probably mark the eggs now under her and collect any fresh ones. You could collect some fresh eggs, mark them and put them under her while tossing all the old eggs now under her, but I think I'd just leave the eggs she already has.