Yes, thats fine that you only want to do 15, you can any number you like, If it were me I would spread them out a little, not for any reason just that I would !! Good luck and hope you have a great hatch
It would be a good idea to add some bottles of water or baggies with water in them, something water filled and sealed to help keep the heat regulated. It makes a big difference! I keep mine towards the center of the bator when I have a small hatch, so they get the benefit of being surrounded by the heating element.