Having 26 torpedoes in the game, I think, is ok. I know U-boat.net says 24, and I pondered this really hard. I created a thread about it and I was then shown a report of from a Type IXD2 that actually carried 27 torpedoes.
After some searching I again found the report. It was U-199. Here is her torpedo loadout:
Torpedoes – 27 carried, stowed as follows:
12 air in upper-deck containers (six on either side).
4 electric in bow tubes.
4 electric in bow compartment bilges.
2 electric in bow compartment floor plates.
2 electric in stern tubes.
3 electric on after torpedo compartment floor plates.
Here is the rest of the report:
http://www.uboatarchive.net/U-199INT.htm
Type IXD stats:
http://www.uboat.net/types/ixd.htm
After reading that report, I just came to the conclusion that having 26 fish in the game is acceptable; especially after seeing that they actually were able to cram 27 onboard.