There aren't quotas for any particular country or region. The number of students in any particular category largely reflects the number of students from that background who choose to apply.
A bachelor's degree and strong English skills are the minimum requirements to apply for a Wharton MBA, and although Africa's population is large, as a continent, it often struggles to provide even primary level education to its people.
Sadly, the number of African students at Wharton reflects the relatively small numbers of Africans who choose to apply. However, I note that a number of African countries have experienced historically strong economic growth rates over the past decade, and hopefully increased economic growth rates will increase the number of Africans who are both qualified for, and see value in, a Wharton MBA in the future.