The eggs she's going to hatch need to all be placed at the same time, or they won't hatch together. You want them all to hatch in about a 24 hr period, otherwise you'll have mom getting off the nest with the new babies while others haven't hatched yet, or worse, are halfway hatched, then they get left to die.
Mark the eggs you give her, so you can check under her daily and remove any new eggs that other hens may have laid. They will often crowd in and do that, or get in the nest while the broody is taking her daily break to eat, drink and poo.
I use a black Sharpy permanent marker, and draw a circle all the way around the egg so I can see the mark no matter which way it's turned.
If you have more than one hen broody at any time, start the eggs on different days, and write the date on the eggs as well, so you can tell them apart. Hens do sometimes move the eggs around. Weird, but true.