When initially setting up the properties of a Panel design, namely, when in the Panel Wizard. However, if changes to the layout of said Panel need to made after leaving the Panel Wizard, then the Reset Panel Layout dialog can be used instead of restarting your entire panel design.
The Reset Layout option can be used from the context menu (when nothing is selected) to change the number of PCB instances and the gap between them.
Shortcuts
Default Menu: None
Command: Reset Layout
Locating this option
Available from: Context menu > Reset Layout option
Using the Dialog
Enter the number of columns and rows that make up the layout of the panels. Below this, are the fields for how large a gap there should be, on the X and Y axis, between each PCB, specifically in terms of the design’s current default unit.