Here's a sample of a basic structuring exercise you could follow:
Define your business as a set of focus areas, which when put together cover your entire product offering. Those would become your tribes.
Think of each of your existing agile teams: which business domain does it belong to? What is its particular mission? Place each team into the relevant tribe.
Imagine you have the resources to spin up the next three to five teams. For each, think about what its mission would be, and what tribe it would best fit in. This should give you a general idea of the hottest areas around which to focus your scaling efforts.
Create a diagram describing your tribes and pods, including possible future pods. Revise as the organization grows, as missions change, and new teams are added.
"When I joined Electric, the top priority was to reshape the Engineering organizational structure in order to prepare the team for scale. To deliver on Electric’s growth objectives, I require 60% growth in headcount this year. However, before doing so, improving the existing structure was critical so that we would be expanding on top of a scalable foundation.
The challenge was twofold:
Structure the teams in a way that best serves existing business and technological missions.
Devise a high-level structure that allows for easy scaling, optimized for adding new teams into appropriate logical spots as soon as new resources are available.