The shortcut bar, where items and skills can be registered for quick operation, initially displays 8 slots, but it can be expanded with simple controls.

[Expanding Shortcut Slots]

Press the ^ icon located on the left or right side of the shortcut bar at the bottom of the game screen, or
press the default shortcut keys 'Q' or 'E' to expand to 2 rows.
Swipe the shortcut bar left or right to switch pages.