I agree she can handle 10 to 12 eggs. A good number is 8 on a first time hen to make it a little easier on her.
As to staggered hatching....it really depends on the hen, and it is a risk. A 2 day delay is not such a big deal, that is pretty normal, as the chicks really don't get out from under mom for the first 2 days. Nature has created food/energy stores enough in the chick from the absorbed yolk so that they don't need any food or water those first 2 days to allow for natural delays of the eggs hatching.
After that, the mom gets up and begins to teach the babies to scratch, takes them to food and water; therefore, there is a risk that the mom will either stay with the unhatched eggs and not take care of her hatched babies, or she will get up and take care of her babies and not sit on the eggs. Some hens handle more delayed staggers just fine; others don't.
You can help your hen by placing food and water close to the nest, but not so that it can tip into it. That way mom and babies can get food and water easily and it may help mom stay on the nest if she doesn't feel so pushed to take care of the hatched babies. I'd also make sure the babies can't wander far away from mom, another stressor that might cause mom to move.
Five days is the maximum delay I'd personally risk as I've witnessed more than that delay in age of chicks can cause problems as they are at such different stages...at a week old, those chicks are running around the playground while the little "infants" still need a lot of cuddling and naptime care with the hen sitting down which can make for contradictory needs and cause some losses.
Again, it depends on the hen. Some are more flexible than others just like some human moms handle a house full of all ages easier than others.
Good luck.
Lady of McCamley