When your StreamYard Studio is at capacity and a guest tries to join you backstage, they will see the following message: "This studio is full."
When someone is trying to enter and the studio is full, the host will see this message.
In order for someone else to join your studio when it's full, you will have to remove a guest from the studio. To remove a guest, click the 3 dots on their webcam card and click Kick From Studio. Then additional guests can refresh the page to join your broadcast.
Note: you can have up to 10 people in the studio on the Free and Basic plans, 12 people in the broadcast studio at once on the Professional Plan, and up to 15 people on the Premium and Growth plans.