Agree with above. I think you can store hatching eggs up to 2 weeks or so (don't quote me on that!), but it's best to wait until one goes broody.
When hen looks broody, I keep a close eye on her. My criteria is spending 2 nights in a row on the nest. Until she does that, I don't give her the eggs I want her to hatch. I usually have some marked sacrifice eggs to tempt her and for her to practice on. If she's thinking about going broody, I start collecting the eggs I want to hatch. Once she's been on that nest 2 consecutive nights, I place the eggs under her. If you don't have all the eggs you want to give her yet, you can wait another day or two before putting eggs under her. Place the entire clutch at once, don't give her a few eggs then add more later. Doing that would give a staggered hatch and she'd be off the nest before all the chicks hatched.
I usually put 6 large fowl eggs under my bantam cochin. I did have one hen hatch out 8 large fowl chicks, that was during warm weather so I was comfortable giving her a few more eggs.
