You can level 10 Heroes at once up to the maximum level.  Start by tapping the, "Mass Level Up" button on the screen where all of your Heroes are displayed.  This selects your ten highest-leveled Heroes, who are not yet at the maximum level set on the server.  Choose the level you want these ten Heroes increased to by using the slider at the bottom of the screen.  Confirm the action and those ten Heroes will be automatically pushed to the level you selected on the slider!