Depends on how many your bator holds/handles. My little bator is set up with egg holder for 42 and I hatch 38 regularly. I like a few holes empty for humidity release or something. But I have better hatches when I only put in around 34-36.
I think each bator and season has its own magic number. Depends on weather, area temperatures and area humidity, etc. I have better hatches in the Summer than in the Spring. Most people have better hatches in the Spring. Just depends on a lot of things.
Oh ya, how many chicks can you care for at one time. My inside brooders will only hold the above and I actually prefer about 24-25 chicks at one time. That's for the 1 week that I brood inside the house. Then they go to their brooder house that will comfortabley handle up to 50 chicks at a time before I move them to a grow out house.