How Many Menu Items Can Be Placed Before the "More" Tab?
A menu is adaptive in each design, i.e. it adjusts to the screen resolution of a device the website is viewed on. The number of items that fit in before the More tab in the site header or the drop-down menu in the Menu widget depends on the device screen resolution, selected design, and length of the entry names.
In the Menu widget, you can choose to move items that don't fit to the next line in the design settings.