This paper presents a mathematical model for the estimation of the key performance indicators of the taxi markets in urban areas. These key parameters are basically the demand and supply for taxi services, the waiting time of users and the cost of the different involved actors. The identified actors are the drivers, the taxi users and the city, represented by the other drivers and the inhabitants. Optimum values for the taxi supply are obtained from the mathematical formulations depending on the demand level and the size of the city, presenting the users waiting time and the unitary system costs related to this optimum fleet size. The model is applied to two modes of operations, hailing and dispatching market. Conclusions are drawn for the best type of market for each demand level and city size.