Can I put multiple playground files into one Xcode workbook?

You can right click on your top level file and select the "New Playground Page" button, which will give you a separate page.

The below image shows the new page created after you click that button. "Untitled Page" is the initial page, and "Untitled Page 2" is the new page.

You can create either Empty Project or Workspace to manage your multiple playground files under one roof.

1. Empty Project:

• File -> New -> Project
• Choose Other and Select Empty

• Create/Add Your Playground files to the workspace by right clicking on the Project Navigator

2. Workspace:

• File -> New -> Workspace

• Create/Add Your Playground files to the workspace by right clicking on the Project Navigator