ABSTRACT Since the European ‘migration crisis’ in 2015, various actors in politics, media and science have called for better data as a basis to efficiently manage migration. This includes a rising demand for more reliable, timely and comparable statistics on the international movement of people, especially on the African continent. International Organizations (IOs) have responded by offering various forms of ‘capacity building’ to their member states. The article looks at the development of statistical knowledge production in Senegal and the Gambia, exploring how interventions by IOs have changed statistical data practices in West Africa. West Africa is a particularly interesting case because, until recently, most post-colonial states did not generate census or survey data on questions of international migration, citizenship or ethnicity. Drawing on recent works in Science and Technology Studies on the performative power of numbers and categories, the article argues that changing data practices have enacted multiple conflicting migration realities in West Africa. Looking at IOs’ epistemic interventions in Senegal and the Gambia, the article offers insights into the ontological politics of counting and categorizing migration in a post-colonial context. It also contributes to a better understanding of the co-production of statistical knowledge and programs of migration management.