Chickens don't count to well.
Broodiness is a hormone driven need to sit. For some gals they will want to sit every 3rd egg ish.. even if eggs are collected every single day and nothing is left inside the box and they are booted out. Those ones will brood their life away on no eggs... in my experience even after you let them sit on and raise a clutch.
If I do want a hen to hatch a clutch.. I mark all the eggs to be left in the nest with sharpie and collect any new deposits every EVENING at lock down. Eggs should also not be laid at night.. so I would suspect your single egg is deposited earlier in the day. BUT... YOUR gal could be an anomaly.. I've seen roosters egg sing, hens grow spurs and even crow! So sometimes there are exceptions. Noting that some of ladies have started sitting and still had an egg or two finishing their trek out.
They usually do not start sitting until laying their last egg of the clutch as it starts development and would make some chicks hatch earlier than others. Staggered hatch is a recipe for disaster in nature.. as there is a window within the which the sitting hen has to decide take the early hatchers in search of food and water and leave the still developing eggs. Or stay on the eggs and hatched chicks might starve or dehydrate. The yolk sustains them for a period before it becomes an issue. But ultimately later hatchers are weaker, have a harder time keeping up as they're behind developmentally compared to others in the same clutch. Again in nature, survival of the fittest a recipe for death. This is just informational for descriptive reasoning of what's going on. Now that sitting hen doesn't mind letting others add to her clutch. She may not have a choice if she's lower in the pecking order in home set ups. Some will consider new depositors as an opportunity have their nest tended while the go run errands (eat, poo, etc). The better sitters make it back to the right nest instead of nest hopping.
Weather you break her or not depends on if you have room (or plans) for more chicks and eventual cockerels. If it isn't convenient for
me and the rest of the
flock then I might break her this time and plan to let her raise a clutch next time... alternating between hens so they both (or all, I've had broody's coming out my ears before like it's contagious and no longer keep Silkies for that reason.
) get to fulfill their natural roles but still maintain an appropriate stock load to keep antics, poo, parasites, crowding, etc to a minimum or down to very manageable levels according to the pasture I like to keep, which will vary for everyone depending on their soil type, energy level, property size, weather pattern, goals, resources, and even individual flock dynamics.
Sometimes I adopt sexed feed store chicks to a hen (after sitting a couple weeks) if they have some under a week old in another breed I'd like to try. I personally always break them if they're pullets as their bodies and minds are still maturing and I figure if they're broody now they will again. But that just MY preference!
Okay so let me see if actually have this right, now that I've read your post again.. Are you leaving the egg that you see in the morning.. and it's the same egg that she's sitting on that has NOT been collected.. or is it a NEW egg after you collected the one you found her sitting on in the morning? Sorry!
If you want her sit on a clutch of eggs.. collect them and tuck them under her all on the same day.. (why I told the staggered hatch story) I might be distracted right now, check back on ya in bit.
