I had a program that had 9 relationship managers that owned their client products end to end. Each relationship manager had anywhere from 2-4 clients. Each client had a unique customer application (similar functionalities, but separate codebase and database).
When standing up the teams, there was only 14 developers and 6 testers. They supported roughly 30 custom applications from windows mobile 6.5, iOS, android, and webapp reporting.
DEVATION: 3 scrum teams with 3 product owners on each team with their own clients.
We had to do a lot of coaching and trust building with the POs to manage a team backlog with sometimes 12 clients of work, but they made it work. The POs had a client priority order, had regular stand ups among themselves to prioritize work, and overcommunicated anything going on to avoid confusion or conflicting backlog priorities.
It was probably one of the most eye opening moments seeing the POs coordinate and self organize like they did. They made a ton of custom Jira board and filters to allow the right views to prioritize their work, then had an overall team board they collaborated on the prioritize the team view so they did not feel confused. They also managed their "client WIP" to make sure the team wasn't sporadically working on 6 clients in a sprint.