As for the number of cannons on ships, the number you find by using the damage code is extremely low. Around 10-15 on a behemoth ? Even if you consider it's only for one of the 4 sides of the stone castle, that's still a joke. 40-60 cannons total on a "hundreds of yards long" castle ?
Let's look at real-life ships. Biggest ships in the 17th century, Man'o'wars, which should be the equivalent of the Earthdawn Galleons, were about 55-60m long (for 15 width), and had between 70 and 120 cannons (half on each side of the ship).
I tried to find a way to simulate a number of weapons which would seem plausible based on that, on which wouldn't change the damage codes too much. It's not very simple calculating it, but here's what we use for the moment (I don't think it's the best possible, but couldn't get a better idea):
You have a base damage code (7), then each weapon you fire after the first one add a little to that base (1 for fire cannon or spear thrower, 0,5 for catapults or balista). You then check how many weapons you allocate to each target, and roll one attack for each target. To limitate a bit, we've set a limit to 20 weapons allocated on a single ennemy (which pretty much only limits very big ships).
Military Vedette would be heavily equipped, with 9 cannons per side (Firepower 16)
Galleon & Kila would have 40 cannons per side, so 80 & 160 cannons total (Firepower 27 since there's the limit)
One of the problem of those more realistic values of number of cannon is the cost for player-ship. The number of kernel of True elements needed for firing is much more important, which can be problematic, as players don't have the logistics of a military nation to supply them with True Air & True Fire.