Hi Shona,
Your official limit is 2 litres per person of wine or spirits into SA.
3 X 750ml regular size bottles per person would be close enough.
As MK says the wine (and spirits) in SA are so cheap and of such good quality that it is rare that anyone actually wants to bring it in rather than buying there.
It is far more expensive in duty free anywhere versus normal retail in SA, so it is not something customs is usually looking for.
Have never had any sort of problem with customs there in 6 recent visits and they don't seem to bother tourists at all.
Good luck.