Originally Posted by
okeyigiveup
Both Bow and Longbow have the Volley Fire special rule.
So usually a unit can shoot with the first and second ranks + half of the rest.
In addition High Elves have martial prowess, which let lets them shoot with three ranks in lieu of two.
So a unit of Archers / Sea Guard can shoot with the front three ranks + half of the rest.
Which mean : 10 (10+0) for 10 (5x2), 15 (15+0) for 15 (5x3), 18 (15+5/2=>15+2.5=>15+3) for 20 (5x4)
If you really wanna maximise the shot for a unit which is more than 15 models of High Elves, you may go with wider facing units.
But you may also think about the rank bonus and mobility of the unit before doing so.