When a miniature adds a weapon that has the Spray keyword to the attack pool, that weapon contributes its dice a number of times equal to the number of miniatures in the defending unit that are in LOS of the miniature using that weapon.
For example, if an AT-RT with a flamethrower attacks a unit of 3 Death Troopers, since the flamethrower has the Spray keyword and an attack value of 2 black die, the flamethrower contributes 6 black attack dice to the attack pool.