An analytical characterization of the neutronics of a hybrid blanket is used to determine the regimes of tritium production. A set of fundamental parameters is identified as the basis of this characterization, which enables one to set an upper bound for the number of tritium atoms produced per nuclear fusion. This upper bound depends mainly on η - the mean number of secondary neutrons per absorption in the fuel of the blanket - and it is found out the requirement of a value η>l for reaching tritium breeding.