The JNB~CPT route is indeed a busy one. There are five carriers on it and the JNB departure board for Tuesday 10th Nov shows 49 departures. These range from MD80s, through 737-300/400/800 and SAA often run much large a/c, certaily 747s (in the past) when they have a big sporting event.
At morning peak, SA have a CPT rotation every 25 minutes. It is easy to turn up at either end and just shop the counters to see who has the best fare and how many minutes you are going to have to wait. Doubtless someone has calculated the seats for an average day.