Auto Rank Up allows a feature which allows players to perform several rank ups at once. This is limited to 10 at once. When tapping the rank up option, a pop-up window will appear, providing a preview list of all the rank ups which will be performed.

You will be able to uncheck the rank ups you do not wish to perform. Once satisfied with your selection, simply confirm this action, and the selected heroes are automatically merged without any further action needed.