A simulation study of a silicon pixel multiplicity detector proposed for the NA57 experiment at CERN SPS has been carried out to evaluate its capability to measure both the total multiplicity and the net baryon content created in Pb + Pb collisions at 40 and 160 GeV/c per nucleon in order to get information on the specific entropy produced in the reaction. A method of charge assignment is proposed and discussed in detail and the results of the simulation are presented. A benchmark of the method using computer-generated events is also performed.