I've done Fairies in 3 different worlds. Twice I used 4 Day Farms and 4 Night Farms, all of them fully upgraded. The 3rd time I used 4 Day Farms and 3 Night Farms, all fully upgraded. I really wanted a 4th Night Farm, but could not make the space for it work in that city's layout. It was a little harder, I really had to pay attention to what the Night Farms were making cause I only had 3 but it worked out. Of the 2, I think it is more valuable to have an extra Day Farm than an extra Night Farm.