The tempo refers to the average tests you run in any given week. This number can be as small as 3 (typical for smaller startups), but can be as much as 20 or more for larger teams with plenty of resources. This number can always be adjusted later if you determine it needs to be increased or decreased.
To set the weekly tempo, click on "Dashboard", then "Edit Tempo" next to the weekly tempo number.
Use the slider at the bottom until you see the number you'd like to set, then click "Set it" in the upper right-hand corner.