For storm window units, the part that you’ll be counting is the unit that needs to be removed in order to be cleaned. In the example below, you would count the highlighted part as 1 storm window unit.

You then need tally up the window that’s left behind.
In this example, there are: 4 divided panes (in blue) and 1 regular pane (in white).

So if this was your window, and you were texting us to get pricing, you would say:

“I have 1 storm, 4 divided, 1 single.”