I prefer 15. Count me among those who believes money was more of the actual reason in boxing changing the number of rounds and weigh in procedures than fighter safety.
Deaths and near-deaths have happened in fights ranging all over in length. Some went more than 12, most notably, Mancini-Kim. They've also happened in fights than went 11, 12, just a few, etc...Chavez-Johnson went 11 rounds. Eubank-Watson 2 and Pintor-Owen went to the 12th. Should we then go to 10 rounders?
Gonzalez-Velasquez went 8, Hart-Denny went 10, Montiel-Alcazar and SRR-Doyle went 6, etc...They range all over in length and tragedies will always occur due to the nature of the sport. Ray Mancini has said there is no evidence that the change to 12 rounds has resulted in fewer deaths and serious brain injuries.
Procedures outside the ring are more important I think. More strict tests to be able to get a license, for instance. I know it's difficult, but having doctors around fighters more.
I hate the WBC, but I do like the mandatory check-ins for making weight.