There is no "Bantu Alliance" but there are three supra-regional groups: the SADC and EAC and ECCAS that make up the Bantu countries per the linguistic map. I can go and provide references for all 19 predominately bantu countries and the 4 islands as well. Let me know what you think is best. But the three organizations, removing overlap and accounting for non-Bantu groups is sufficient. If we have a page on Bantu peoples there should be an estimation as to their population. If there is an easier way let me know.