I like to start my hens on 12 eggs. This is because when we take out the eggs that aren't fertile (Say 2), take out the eggs which died along the way (Say 2 more) you have 8 chicks. That is what I call a nice hatch.
But it depends on each hen and her size. We had a white banty mix go broody and I wanted to start her with 12 but she couldn't cover them enough so I brought her down to 9. I should still get a good hatch from her.
If a hen is going to sit on a nest for 20-21 days, I expect at least 50% hatch rate out of set eggs. I will not accept a hen hatching one chick and then leaving the rest of the good eggs, just because she had one baby. (Although normally we let our hens do the hatching in a confined box so she can't get out. I just have to check in on her during the hatch day so I know when she's ready to come off.)
Last hatch we did with a hen we set 11 eggs (She is a banty but covered them well), removed 1 that was infertile, and got 8 chicks. Not bad I would say. (We put the other to eggs in the incubator but they never hatched.)
Edit: I don't use Sharpies to mark my eggs. I use pencil. Don't Sharpies clog the pores?