Major hatcheries very, very rarely have extra. I know that I try to buy any extras that the hatcheries I deal with have, on their hatch days. Other hatcheries have other people like me that do this. It is like extra "processing" to deal in orders of <5 chicks, and that is a fact, though other reasons apply, I believe that to be the main one. Small hatcheries behave the way you describe, for example JM hatchery. However, the large hatcheries hatch what they can of each breed, as per the market, and typically sell out by a hatch day. For example on the market, a hatchery may set 10X the Barred Rock as they would the Black Sumatra, as BR are a common breed that many people are familiar with and therefore higher volumes are purchased. Those facts taken into consideration, the hatchery plans the numbers of the different breeds to hatch, and set a flat rate of those.