Humans and animals are able to rapidly, and with reasonable accuracy, estimate the number of objects in a visual scene. Visual-evoked potential studies have described a sequence of functionally distinct stages associated with numerosity perception. Recently, a specialized cognitive-perceptual system for the numerosity estimation for human stimuli (NEH), distinct from that for non-social stimuli, was revealed using virtual reality, revealing a stable NEH overestimation bias (ie perceiving more people than were shown). This bias was further increased when a social hallucination characterized by the false perception of another invisible person (presence hallucination) was induced robotically and repeatedly prior to NEH trials. However, little is known about the brain mechanisms of NEH and robot-induced presence hallucination. Here we combined virtual reality and robotically-induced presence hallucination with visual-evoked potentials to investigate the neural mechanisms and processing steps of NEH. We report that NEH induces numerosity-related components as observed for non-human numerosity stimuli and, critically that experimentally-induced presence hallucination selectively modulated the P2p component, whose amplitude correlated with NEH overestimation magnitude. This effect was localized in left extrastriate cortex, showing that robot-induced presence hallucination are integrated with NEH processing during the P2p time period and relying on social numerosity mechanisms in extrastriate cortex.