Along with Darkwing, I always assumed that the 15 launchers were mostly there as being simple, direct fire models instead of the "shotgun" style launchers that the Galaxy
class generation use. They're also angled in ways that make the launchers appear to be geared toward engaging multiple targets in some sort of saturation fire mode.
The CGI model is kinda weird on the placement of the torpedo tubes.
There's anywhere from 2 to 4 launchers on "flank" of the saucer that faces directly to the sides, and another single launcher that's located on the engineering hull. In addition you have openings for 4 forward launcher bays, and 2 rearward facing ports that have 3 tubes each. Then there's at least 2 more openings on the "under" side of the pod. That adds up to something between 17 and 19 launchers, which is consistent with Alex Jaeger's statement that there's 15 photon and 2 quantum torpedo tubes on the Thunderchild
The torpedo tube pod is supposed to be modular and replaceable and Alex Jaeger also later noted that the "carrier" is more for carrying shuttles, workbees and runabouts than "fighters".
I think the best way to rationalize it is that Akira
is meant to be both versatile and easily configurable for different mission profiles. So the emphasis on having a larger number of auxiliaries on board with the "through deck" saucer (with most of the saucer volume dedicated to storage and hangar space) is to make it really easy to swap mission payloads in the form of shuttles and runabouts.
The torpedo tube pod plus the side facing launchers suggest they're meant to basically spray torpedoes in different directions in independent firing modes, so maybe the ship is designed to engage a lot of small targets at once using a large number of single firigng torpedo launchers. Perhaps this was because Cardassians were using big numbers of Hideki
style ships and during the Dominion War the prevalence of large numbers of Jem'hadar attack ships meant that having the ability to lob torpedoes in a lot of directions at small targets was a boon.
There's 8 in the saucer plus the "pod" which has at least 9 openings for torpedoes. Jaeger stated in a later interview that the ship CGI model has 17 total launchers, totalling 2 forward firing quantum torpedo launchers,