A collection of Object content.
If you've only got one type of Object in your game, Playbooks are a simple way to bring all the relevant content into one place.
Deciding how many Playbooks to include with your game is a question of variety and scope:
Within the broad category of Objects your game is designed around, how many distinct archetypes do you think there are? In Artefact, the nine Playbooks all represent magical items, and (for example) a Weapon is clearly different from an Instrument in some fundamental ways that the game wanted to represent. But the difference between different kinds of Weapons (e.g. axe, sword) was not considered great enough to warrant unique Playbooks.
Also, of course, there is no hard stopping point for how many Playbooks you might create—but at some point you'll run out of time, money or interest!