A Dashboard can consist of a single collection of panels (a base page) or a series of tabbed pages, each with its own collection of panels. Dashboards can be configured by your developer to be static or adjustable.
A tab's Settings menu, shown below, appears when its gear icon is clicked and allows you to add panels to the tab, change the number of columns being used in the tab, to rename the tab, and to remove the tab.
Dashboards can also be configured by your developer to limit the number of panels and tabs. The Add Panels pop-up panel indicates whether or not your Dashboard is configured to restrict the number of panels and tabs:
To add a new Dashboard or Tab Visual, you must first remove existing panels/tabs that are exceeding the count, based on the limitation.