Atlassian doesn't sell Cloud licenses for instances beyond 2,000 users. At that point, you need to dial up your own "on-premise" Data Center version yourself on AWS in order to scale further - not to mention federated use cases, which Atlassian themselves employ, e.g. a public-facing instance for tracking suggestions and public bug reports, and an internal instance for development itself.

Atlassian Cloud hosting is for small players.