All bound booklets must have a page count that is a multiple of 4, for example: 4, 8, 12, 16, 20, 24, etc.
Pages of a booklet are created by printing four pages (2 in the front and 2 in the back) on one sheet of paper and then folding that sheet in half. In the illustration shown from Opus Design, you can see that each page is connected to 3 other pages. Because of the way a booklet is produced, you can’t have a random number of pages. It has to be a multiple of 4.
Exceptions to the rule
The only exceptions are wire bound projects, spiral bound projects, and perfect bound books.
Because of the binding process, these types of specialty bindings don’t involve folded sheets. These bindings require page counts to be multiples of two, instead of multiples of four. Perfect bound book binding consist of printing 2-page spreads and glueing them to paper board. Spiral and wire binding are 2-page spreads that are cut and drilled before being bound by spiral or wire.