What is the right number of Organizational Units I should define for my organization?

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The right number of OUs will vary based on your organization. It should be equal to the number of organizational segments that you have. If there are parts of your organization that are not involved at all with the governance of your cloud computing presence, they do not need an OU defined. Any segment of your organization that has some involvement with the cloud journey should have a corresponding OU in your organizational hierarchy.