Question. I've got bantams (2 cochins, 2 OEGB & 1 silkie) for my broodies. IF they do go broody, how many large sized eggs is the recommended number to set under them?
OEGBs? Two. Three if you have a good nest.
Cochins--bantam or large? Can't speak for large, but Sylvie can cover four, on a good nest. Possibly more, but she doesn't like a lot of eggs. (Yes, I named myself after a chicken.)
My last silkie broody hatched out eleven of a dozen, but four were bantam. I wouldn't generally go above ten, but silkies can vary so widely in body type (depending on hatchery) that it's hard to say for absolute certain.
I'd invest in some rubber pans. They're great for shoring up the sides of nests.

No idea on Turkeys. Sorry.