* Definition of "afford": This includes not just tuition fees, but also the cost of uniforms, books, transportation, and other school-related expenses. A family might be able to afford tuition but not the other necessities.
* Geographic location: Access to education and its affordability differ significantly between developed and developing countries, and even within countries themselves. Sub-Saharan Africa, for example, has considerably higher rates of out-of-school children than North America.
* Data collection challenges: Accurate data on school enrollment and the reasons for non-enrollment is difficult to gather in many parts of the world, particularly in conflict zones or areas with weak governance.
Organizations like UNICEF and UNESCO regularly collect and publish data related to education access, but they don't typically provide a single global figure for children who can't afford school specifically. Their reports often focus on broader issues like overall out-of-school rates, which include children who can't afford school as one contributing factor among many (child labor, distance to school, discrimination, etc.). To find specific numbers, you would need to look at reports focusing on individual countries or regions.