In order to design a spine there is a minimum page count to ensure the spine is wide enough to add text.

Minimum page count for designing a spine- Paperback Book
Matt 90 gsm (b&w) - 70 single sided and 139 double sided
Matt 100 gsm (colour) - 76 single sided and 151 double sided

Minimum page count for designing a spine- Hardback Book
Matt 90 gsm (b&w) - 44 single sided and 87 double sided
Matt 100 gsm (colour) - 48 single sided and 95 double sided