Twenty one countries, and the exact list matters far more than the number. Several are ones nobody expects to see on an Africa plan. Some of the continent's most visited destinations, South Africa among them, are not on it at all.
The full list: Algeria, Chad, DR Congo, Egypt, Gabon, Ghana, Kenya, Madagascar, Malawi, Mauritius, Morocco, Niger, Nigeria, Réunion, Rwanda, Seychelles, Tanzania, Tunisia, Uganda, Zambia and Zimbabwe. Check your route against that before you read another word.
The surprises sit at both ends. Réunion is on the list, which is odd, because Réunion is legally part of France and sits in the Indian Ocean. Mauritius, Seychelles and Madagascar are on it too, so several island holidays are covered by an Africa plan. Chad, Niger and DR Congo are on it, which few consumer eSIMs bother with. Being listed is a coverage fact, not a travel recommendation, so read your government's advice for those three separately.