The NumberOfSolutions command computes the number of solutions of a system over the algebraic closure of the coefficient field, including multiplicities. A zero-dimensional system has a finite number of solutions.

Let G be a Groebner basis for the ideal, then the number of solutions is equal to the number of monomials not divisible by a leading monomial of G.

This function is part of the PolynomialIdeals package, and can be used in the form NumberOfSolutions(..) only after executing the command with(PolynomialIdeals).  However, it can always be accessed through the long form of the command using PolynomialIdeals[NumberOfSolutions](..).

Cox, D.; Little, J.; and O'Shea, D. Using Algebraic Geometry. New York: Springer-Verlag, 1998.