How many eggs can a light sussex sit on?

A full sized light sussex hen could potentially sit on over 20 eggs. However, she would struggle to cope with that many chicks if the hatch was a successful one. 15 is a good number.

Also make sure u mark any eggs that are underneath her, she will be stealing eggs left right and centre from your other hens trying to make them hatch too. there's a high chance these eggs wont hatch as they would be at an ealier stage of incubation when the majority of the chicks hatch. its very annoying when a broody hen steals all your eating eggs and you cant tell the difference between them.