Are most South Africans white? And besides South Africa what other countries in Africa have mostly white people?

Most South Africans are Black, according to American lingo, because Americans include part-Black as being Black.  There is a significant White minority that used to run the country, until around 40 years ago. But now all racial groups can vote, and the Whites have little influence.  

No, most South Africans are not white, white South Africans consist of about 5 to 8 million people in South Africa out of a population of roughly 60 million people. No African country has mostly white people, however Namibia and Zimbabwe have sizeable white populations

No, "white" people make up less than 10% of South Africa's population.