If you bought an incubator that is a CT model, then no, your egg racks do not have to be completely filled. Our CT models feature our egg racks that snap into our Conturn egg turner. The egg racks move instead of the entire tray, preventing eggs from sliding or moving during incubation. Conversely, our HB model incubators come with adjustable setting trays. If these setting trays are not filled fully, there is the possibility that your eggs may slide or roll.