When estimating your locker requirement, it may seem that you would want to have only one starter per row, and the rest as adders. While this is a valid method that is the most economical configuration, you may want to consider including more than one starter per row.
Remember that a starter locker has two side panels, and that an adder has only one side panel, sharing the side from an adjacent locker to close off the open side.
In new construction, the reality is that things can change from what the plans show. Walls have to be moved to accommodate plumbing or some other requirement. That means the lockers will no longer fit in the space provided. If a row of lockers has only one starter, and you have to remove one or more lockers, there will not enough side panels to close off the leftover adder unit(s). That keeps you from using the leftover lockers somewhere else.
If you have more than one starter unit in the row, you can remove one of the side panels from a starter and use it to close off the leftover adder units. The leftover lockers can now be installed elsewhere in the project.
Consider this first layout, with 1 starter, and 9 adders for a total of 10 frames with 11 side panels. If you have to leave off one of the adders in the field, you will not have a side panel to close off the removed locker. What you have left is only good for spare parts.
If you had ordered this row of lockers as 2 starters and 8 adders you would have 12 side panels to work with. If you remove one of the starters, you can complete the row with 1 starter and 8 adders, leaving you one complete locker that can be used somewhere else.