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I also think that the 10-15 max is more manageable for a game. I also tend to think of Fire Cannons as similar to the early western & eastern cannon, not the late age-of-sail iron cannons. But that is an interesting context question, which deserves a bit of research.

The way some ships are described, especially the galleon, just scream age-of-sail, though, as they are triple-masted boats with multiple decks (and even the images... just check the 3rd Edition about Airships, the drawing on page 58 or 71 is pretty clear...). Biggest ships on the Greek/Roman age would have been the Triremes, but they only had one mast, and sometimes two, one of them being a foremast/bowsprit, so not a "true" mast.

Smaller ships are a bit different, like the Troll ships, but those mostly use boarding as their attack pattern, and not just shooting from afar.

I agree that a small number of weapons is more manageable as a gaming factor, but for realistic sake, it's completely ridiculous to imagine that a behemoth, being a rough square with sides of about 250 yard long, would have only 50-60 cannons, or a galleon only 10... Those are warships, not cargo ships, they should be packed with weaponry...
Boats who didn't have many cannons, before the 16th-17th centuries, it was mostly because the main attack pattern didn't use that weaponry (they mostly used rams and ranged weapons from the crew, like spears, bows and crossbows). But that doesn't seem to be the case in Earthdawn. Theran and Throal naval armies clearly use fire cannons as their main weaponry (at least it is described that way).

All very valid points, and a lot to mull over. Namely, how to make it manageable AND realistic.

An idea I had to handle large quantities of guns is something I call "Volleys". I'll get to that in a moment.

Basic, one-to-one scale combat would be as I've stated before. Ship weapons have normal stats, plus Reload. Perhaps Reload should be X/Y, where X is the number of rounds to reload, and Y is the difficulty to "Rapid Reload", which is a STR (Y) test (example: 3/10, 3 rounds to reload, difficulty 10 to rapidly reload). This difficulty could be a flat number for all Weapons instead of a different number. That would be easier.
Rapid Reload: standard action; make the test; each success reduces the Reload by 1 (min 0); failure adds 1 to Reload; GM discretion for worse results, like blowing up in Player's hands or from dropping it. Suggest that players be made aware of the possibility of such bad results before trying. Suggest also mechanisms for such bad results, like Rule of One or miss success by 5.

Now, Volleys. 2 to 15 Weapons can be combined into a single Volley. A Volley represents the combined attack of the weapons, which is just the summation of their individual Firepower ratings. So this would operate the same as what has already been discussed. Most ships would likely have a minimum of 1 Volley possible (15 or fewer Weapons),  but a Theran Behemoth might have 4 or 6 Volleys minimum (60 to 90 Weapons) or something.

Volleys can be separated and recombined however is necessary. Each Volley is used as a separate attack, so each requires their own attack tests and damage tests. As such, it's like any creature with multiple attacks. You just get to decide which weapons are included in which Volley. A ship with 10 weapons can have 1 to 5 Volleys (10 weapons to 2 weapons each). It's easier to just use larger numbers per Volley / fewer Volleys.

Volleys are also different in this way: each extra Volley incurs a cumulative increase in difficulty to attack, adding to it the number of Volleys that will have been made, including the new one. So, 1, 2, 3, and 4 Volleys add 0, 2, 5, and 9 to the difficulty. This represents how hard it is to bring to bear Weapons that aren't already pointed at the enemy. It also keeps the damage per round against 1 target down to less crazy levels.

A single Weapon does not count as a Volley, does not increase the attack test difficulty, and is not affected by the increased Volley attack test difficulty.

Alternative rules:
(1) Volleys cannot attack individuals (except Dragons?). I'm not sure I like this idea, but whatever. I'll throw it out there for discussion. Perhaps Volleys cannot do Called Shots, which are required to attack a person?
(2) On-the-fly changes to Firepower calculations should be easier than having to remove the respective weapons, adding them up properly, etc. For quicker reconfiguring, just spread out the Firepower total over each Volley with the minimum Firepower per Volley as 10. With my proposed Firepower ratings, a Ballista would have a Firepower of 4 (12 ÷ 3)*. 2 of those would be 8, so 10 ought to be a good number to cover most combinations of 2 Weapons.

*Reminder, Firepower would be Damage Step divided by (Reload plus 1).

Anyone following this thread may want to check out the for this topic.