Twenty nine countries, and the exact list matters far more than the number. Several are ones nobody expects to see on an Africa plan. About a dozen of the continent's most visited destinations are not on it at all.
The full list: Botswana, Burkina Faso, Central African Republic, Chad, DR Congo, Egypt, Eswatini, Gabon, Ghana, Guinea-Bissau, Ivory Coast, Kenya, Liberia, Madagascar, Malawi, Mali, Morocco, Niger, Nigeria, Republic of Congo, Réunion, Senegal, Seychelles, South Africa, Sudan, Tanzania, Tunisia, Uganda and Zambia. 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. Seychelles and Madagascar are on it too, so two island holidays are covered by an Africa plan. Central African Republic, Chad, Sudan and Guinea-Bissau are on it, which almost no consumer eSIM bothers with. Being listed is a coverage fact, not a travel recommendation, so read your government's advice for those four separately.