It depends on the chicken. Some will sit and then gradually get more and more eggs. Chickens in all don't have a "limit" of eggs they will sit. Often a hen will go broody without sitting on any eggs. Some hens have sat on twenty eggs but it can be hard for them. Sometimes one egg. But on average and for an average chicken, it would be around a dozen.
because they are putting there scent on the nest to say "this nest is mine, leave it alone!" They could be laying the eggs
They left the eggs alone
Yes, a hen duck may sit on a nest with unfertilized eggs. Some hens exhibit broody behavior and will sit on eggs regardless of whether they are fertilized or not. However, the hen duck may eventually realize that the eggs are not viable and may abandon the nest.
The parents sit on them and the nest holds heat.
Geese do not get pregant. They lay eggs and sit on a nest.
Yes, if young or eggs are present.
it left it to hatch alone
Pea hens lay eggs and sit on the nest about 28 days to hatch a clutch of eggs
Tortoise do not sit on their eggs (they are not warm blooded so they can not warm them), they bury them in a nest made in a warm moist place and they hatch naturally.
Because it is in their nature to sit on eggs, even when there are none there. But they need to get up to get the exercise for their legs.
No, they occasionally leave it to get water and/or food
If you leave them about a week then they might sit on them.