I wouldn't worry about having 24 eggs, hand turning is easy, just turn an odd number of times each day, minimum of 3 times.  They'll move a bit anyway when the automatic turner turns the other eggs, just do a few test turns to make sure the bottom eggs are turning as they should with the eggs set in the spaces on top.  I've hatched several times this way, no issues.  Not something to worry about.  Humidity is controlled by how much you close the "window", and the instructions tell you exactly where to set it and how to do it, just keep water topped off AM and PM.  I used about half a gallon of distilled per hatch, works with sink water too, I just wanted to avoid calcium deposits cause my water is hard.  Good instructions come with the unit, very straight forward, and several informative threads on here about the unit.  People hatch in this incubator a LOT.  It's pretty plug and play.