In this chapter, methodologies for estimating the total number of taxpayers in Mexico's tributary system are proposed. The methodologies are based on the theory of optimization and consist of an initial model with differentiated results, a prototype model with constant and differentiated returns, and a generic model for Mexican income tax (ISR). Based on the theoretical results, the models permit to estimate efficiently the expected number of contributors under different scenarios. Moreover, when the estimated data is contrasted with official data, they give satisfactorily results. The proposed models may be even adaptable to the inner conditions of Mexican tributary authority and may become an important tool for the Mexican government in their overall fiscal process.