We propose a conceptual framework to investigate factors influencing the supply of Airbnb units in US urban communities. Based on a sample of Airbnb supply data from 1068 zip codes in 28 major US cities, we apply a mixed-effects negative binomial model to uncover supply determinants. The results confirm the significance of five supply determinant categories, namely, hotel market demand and supply, housing market demand and supply, and short-term rental regulation. We also highlight the respective impacts of supply determinants on three different Airbnb units: entire houses, private rooms, and shared rooms. Moreover, we examine the unique effects of different types of regulation policies. In general, regulations related to tailored legal framework and hostile enforcement significantly decrease Airbnb unit supply.