How many eggs does a hen lay when she is sitting?

An individual hen will go"broody" and will gather a clutch of eggs to brood. These will not always be her own eggs. She will steal them from other hens by rolling them into the nest she has chosen. Unless the hen is broody she will lay her egg and leave the nest announcing loudly to the rest of the flock what she has accomplished. Hens can go broody whether there is a rooster in the flock or not, so no, not just fertile eggs trigger the brooding instinct.