Software mutation is a widely used technique of software testing that consists in generating variants of a base program by applying standard modifications to its source code. One of the main obstacles in the use of software mutations is the existence of equivalent mutants, i.e. mutants whose behavior is indistinguishable from the base program, even though their source code is distinct. Despite several decades of research, the identification of equivalent mutants remains an open problem. Rather than attempting to identify individual mutants that are equivalent to the base, we argue that it is often sufficient to estimate the number of equivalent mutants; also, we argue that the number of equivalent mutants depends on two factors that must be considered in the estimation effort, namely the base program and the mutation operators that are used; in this paper, we explore the impact of mutation operators on the number of equivalent mutants.