There is likely a cap of how much ammo a siege engine may have. Also the number of rockets per volley may be a certain fraction of the total ammo (determined by descr_projectile).

What does your errorlog tell you? Something about out of bounds possibly or invalid number? If nothing, try a few numbers that are only slightly larger than the default to observe differences. The best way of doing that is to increase accuracy in projectile and fire at a wall to see whether there are more rockets (i.e. more damage) and number of volleys.