In a series of interviews with Clelia Tuscano in the 1990s, Giancarlo De Carlo revealed his admiration for Aldo van Eyck and the influence the Dutch architect had on him. Albeit starting with a disagreement during the Congrès Internationaux d'Architecture Moderne (Ciam) that took place in Otterlo in 1959, the relationship between the two will subsequently evolve in a crescendo of mutual esteem and exchange, until reaching a sort of ideal mutual understanding that will strengthen one of the most oriented lines of research within the variegated Team 10 group. The central issue into which the two architects channelled most of the energies they spent at the international level responded to the challenges posed by mass society, or to what has been called ‘architecture of the great number’. This theme acquired international relevance within the Ciam, starting with the success of the North African grids presented at Ciam IX (1953), including the one emblematically titled “L’habitat du plus grand nombre”, and then accompanied the evolution of Team 10 until the XIV Triennale di Milano (1968), dedicated to the “Grande numero”.This essay is grounded on the consideration that, in De Carlo’s architectural evolution, there is a before and an after Otterlo ‘59, a real ‘turning point’, not only because from then on the Italian architect became one of the main protagonists of Team 10, expanding the range of his design tools, but also on account of the theoretical and design potential that the event unleashed in the imagination of the attendees.