I won't be much help but it may give you an idea...
I have 17 Guinea eggs under a bantam sized hen. She is a big bantam, about the size of a 4 month old BO Pullet. She covers them well and they are all doing well.
Wowzer! My old english game bantams can cover about 3 LF eggs!
I put 11 eggs under her, but I have 16 eggs. I wonder if she can cover them all? I'd hate to lose any of these eggs by putting them under her when they should have gone in the bator...