Historical estimates of how many people the Earth can support ranged from <109 to >1030 people. The estimates had widely different assumptions, methods, and purposes. To make "How many people can the Earth support?" into a scientifically meaningful question requires at least 11 basic assumptions. Even with clear assumptions, estimates of how many people the Earth can support depend on interactions of populations, environments, economies, and cultures. These interactions are complex and poorly understood. Hence estimates of how many people the Earth can support are highly uncertain.
Although no one knows how many people the Earth can support, people can do three kinds of things to make life better now and in the future: create a bigger pie (create and use new technologies), bring fewer forks to the table (slow population growth and reduce the material throughput of consumption), and practice better manners (resolve conflicts peacefully, trade more efficiently, and govern less violently and less corruptly). Universal basic and secondary education would help future generations do all three. Education should give children a good understanding of the workings of their own bodies and minds and the bodies and minds of others.
But children can learn only if their brains and bodies work well. Hence universal education requires good nutrition for all children and their mothers. Despite a global abundance of food, nearly a quarter of children under 5 years old are stunted from chronic undernutrition and infection. The reduction in later economic output when children become workers due to malnutrition in childhood is far greater than the cost of feeding all children and their mothers well. Feeding and educating all children and their mothers well is profitable economically and desirable morally.