I have pondered this numerous times, and after seeing an interrogation report of some survivors, 26 fish may not be all that unrealistic.
http://www.uboatarchive.net/Uboatlist.htm
In that site listed above, is an interrogation report of the survivors of a Type IXD. They had crammed 27 torpedoes in it! I tried looking for that report again. I always forget which U-boat it was. I searched through twice, but couldn't find it. I know it is there.