I'm including a map because it always makes threads look nicer (and who would have known where Southern Africa is otherwise?!).
Anyways, provided earlier colonization of South Africa (say, 1500 early), with steady waves of immigration coming in every few decades and with the population naturally increasing very quickly, the area could quickly fill up with people and have tens of millions of settlers by the mid 1800s. Provided that the settlers politically and militarily dominated the region as one united country and did not have to fear foreign militaries or native tribes, how far North can they spread before disease makes further advance impossible without major technological PoD?
Also, what would the likely carrying capacity be in the early 1800s for a fairly advanced European society? If they were mixed race (probably Spanish or Portuguese colony originally instead of Dutch or Anglo) due to intermarriages with the natives, could they spread further North and establish larger populations than pure European colonies with African ancestry, or would disease also be too much for them and most of their animals?