Subitizing is a fast, low-effort mechanism for perceiving the size of small sets: typically up to four or five items at a time. Within that range, perception is rapid, accurate, and nearly effortless. Beyond it, the visual system switches to slower, serial counting and processing. People can extend the range by regrouping larger sets into clusters, letting the brain reuse subitizing at a higher level of abstraction. The capacity appears across sensory modalities and reflects an efficient strategy for processing complex scenes. For evaluation and annotation, it marks a natural limit: judgments of five or fewer items come almost free, while larger sets require a jump in effort.