Breed Clubs decide the classes they want for any show and inform the relevant Show Secretary of these. Smaller shows will have fewer classes as they expect fewer entries but the large shows should have classes for each colour of a breed, often separate male and female classes plus classes for current year bred birds. Access the complete Classification of Breeds here.
Entry dates are stated in the show Schedule and it is important that entries are checked when penning slips are received since it is common for birds to be entered in the wrong class (e.g. large fowl in a bantam class) which causes problems for the show organisers since the pens the birds spend the show in are ordered on the basis of entries which includes size.
The National Show had 1405 different classes in 2010 covering trios, pairs, all the chicken breeds both large fowl and bantam, turkeys, ducks, geese and eggs (click on Eggs in the header bar).
All classes at poultry shows are for pure breeds, including the Utility class. It is possible to request a class for Hybrids but this needs to be done well in advance.