When you purchase the Multibrand add-on for Enterprise, there is a maximum number of brands you can create. This limit helps ensure that the feature is manageable and performant. Brands, more than most Zendesk features, are somewhat resource intensive, so having a large number of them can cause issues in the agent interface and admin pages.
In general, a brand is meant to represent a public facing identity that your organization or company uses when interacting with your end-users. In our survey of customer use cases, it was pretty uncommon for a single group of agents to represent more than a few dozen brand identities, and the brand limit reflects that finding. That said, there are exceptions.
If you find that you need more than the permitted 300 brands, contact the Zendesk customer service team (email@example.com). The team can raise the brand limit on individual accounts, but cannot ensure that there will not be performance issues. Brand management was not built for this kind of scale, meaning it may be a little more difficult to maintain.