I ordered the larger version for the 28 hatched last time. You can raise both the one you ordered and the larger one up as they get older. I have used the small one for 12 chicks before. Are you brooding indoors (or climate controlled) or outdoors?
I'll keep them inside as long as possible but it depends on how many hatch as to how long they are able to be inside. I plan on having them inside until they are at least a month old.