In the fictional boxing world of Rocky, Rocky having 10 title defenses in 4 years is very believable when you look up boxing history, in the 80's Mike Tyson defended the heavyweight title an average of 3-4 times a year and in the 50's Jake Lamotta and Sugar Ray Robinson fought 3 times in a six week period, the reason you only see one fight a year title defenses is fighters want to build up title fights as a huge one night mega event for big pay days now, if they lose a tuneup fight they could lose millions, so they're afraid to take the risk and fight to many times a year.
So in the end there is no reason to fight multiple times a year when you can make a years pay in one night.
People need more time to recover when they get really badly hurt but he really outmatched those opponents and won with little difficulty so he might have only needed a few months between fights. 4 years is 48 months so if he had ten fights that is 4.8 months between them, which is plenty of recovery time if you totally outmatch your opponent and do not get badly hurt.