You can add as many text boxes to a scene as you like.
You can do this either on the 'Quick Edit' screen or the 'Advanced Editing' screen.
Here are the steps to add more text boxes:
Step 1: Go to the scene where you want to add the text box.
Step 2: Under the 'Text' tab, choose the kind of text box you want to add - Headline, Lower Third, Miscellaneous.
Step 3: Find a text box that you like and click '+' to add the text box to the scene.
Step 4: You can customize this text box like any other text box in the scene.
If you want to remove the text box -
Step 1: Select the text box.
Step 2: Hit 'Delete' on your keyboard or click the delete button on the screen.
Here is a short video to take you through the process: