Setting up the development team

Hello, I am appointed as a Scrum Master. The company I am working for is diversifying its working and plans to employ 100+ developers. Scrum suggests that a DT should not have more than 9 to 11 members in a team, and so I referred to the Scrum guide for possible solution regarding how to form teams. However, the guide does not mention anything about how team members should be selected or how to set up teams. I would like to know who decides about the team size? Does the management plan the teams or do the team members decide how large the team size ought to be ideally? Should the PO or the SM suggest a solution?

Posted By - SW