I agree with everyone it depends. I have a homemade incubator. It will hold 28 standard size eggs and will be wall to wall eggs. I hand turn. Then after I have culled out a few duds along the way I always have enough room to hatch. It always works out.
Those turners should hold 41 eggs (6 rails with 7 cups per rail). The newer ones come with a small cover to cover the egg cup right next to the motor because of possible issues with the egg hitting the motor when it rotates toward it.