What's the maximum amount of cards you can have in your sideboard?

In competitions where a sideboard is allowed, a player’s sideboard can include 8 or fewer cards.

According to Rule 601.1.c.1: "A player’s sideboard can include 8 or fewer cards." Additionally, all cards in the sideboard must be valid Main Deck cards, and the standard limits on copies of named cards apply to the combination of your Main Deck and sideboard.