Lap siding runs sideways in courses, so a window splits a few courses. Vertical siding (board and batten and similar) runs the other way: boards stand side by side from the wall's corner, and a window leaves a short piece above it and a short piece below it in every column it touches.
The wall-framing takeoff had only the lap layout. siding.style: "vertical" (CLI and engine) adds the second.
By hand. A 120 in wall, 8 ft high, on 12 in boards is 10 columns. A window from x 40 to 76 in is 36 in wide, but columns are cut at 12 in marks, so it touches the columns at 36 to 84 in: four, not three. Six columns stay full height (96 in). Each of the four cut columns needs a 36 in piece under the sill and a 24 in piece over the head. That is 816 in of board, and packed first fit decreasing into 8 ft stock it is 9 boards.
Limits, all in the README: the piece beside a window is counted at full width (no rip), battens are not counted, a piece longer than the board length is refused instead of spliced, and nothing is checked against a real siding order. Tests are hand-computed only.