The Amazon is home to Indigenous societies that are small and relatively isolated, conditions associated with restricted numbers, systems that count no higher than twenty. Desana have a quinary system that counts to twenty based on the fingers and toes. The Desana numbers eleven through nineteen are unrecorded, suggesting they are “gap numbers” that have not yet been explicitly named. Desana invented a system of numerical signs influenced by Western numbers and the idea of writing. Mundurukú count to about four using “distributed exemplars,” common environmental objects associated with quantities that are small (perceptually appreciable) and reliable (for example, human arms always occur in pairs). Pirahã may not count at all: they have been described as lacking both words and gestures for numbers. The Pirahã language has also been described as lacking recursion, the ability to embed words and phrases that is used in constructing higher numbers (for example, as twenty-three is constructed from two, ten, and three). The extent of Pirahã anumeracy remains contested. Restricted numbers are providing new mathematical insights, as for example, the nature of infinity.