Basically: how many chicks do you want? Two or 3? or 5 …but, there is a maximum and possibly an ideal number. One factor is: how good a mama is your broody? You may not know, but if you do, it makes it easier to decide how many eggs you will trust her with.
This leads back to a general question of what determines how many eggs a bird will sit on.
1 – the larger the losses routinely are to predators, the more hatches are needed.
2 – however large the clutch (which is the collection of eggs) – the mama needs to be able to cover it adequately. So it depends on your hen size.
A normal (non-bantam) chicken clutch averages about 12 eggs. After collecting eggs for 14 days – the fertility goes down . A chicken will collect eggs, 1 every 24-27 hours or so, till she has about 12. Of course, since you collect them daily, she can’t do that…so eventually she’ll just sit. My first 2 had 8 & 7, the third broody 4 eggs which I gave them. My 4rth is sitting on 14 and that looks crowded & IMO, is stretching it a bit, but not all might be fertile. But I have read some try 20 eggs. That is too many. If I get another chance next year – I will give her 10 or 12 eggs, depending on size.
UPDATE: And I gave her 14 and decided to let it go entirely mother nature’s way, no candling. In my opinion, 14 is too many – it was too crowded. One egg got broken after about 9 days. We had a triple digit heat wave the entire last week, and I was not at all sure what would happen, even with fan and mister in the coup run. Final: 2 did not hatch (not fertile). Eight were good hatched. One chicks had a toe problem and seemed weak (the last hatched) …but was fine after 3 days. 2 hatched and did not make it. One pipped but could not hatch as another egg-shell was covering it. In my opinion, 8-10 is a better number.