Zero forcing in graphs is a coloring process where a vertex colored blue can force its unique uncolored neighbor to be colored blue. A zero forcing set is a set of initially blue vertices capable of eventually coloring all vertices of the graph. In this paper, we focus on the numbers z(G;i), which is the number of zero forcing sets of size i of the graph G. These numbers were initially studied by Boyer et al. [5] where they conjectured that for any graph G on n vertices, z(G;i)≤z(Pn;i) for all i≥1 where Pn is the path graph on n vertices. The main aim of this paper is to show that several classes of graphs, including outerplanar graphs and threshold graphs, satisfy this conjecture. We do this by studying various graph operations and examining how they affect the number of zero forcing sets.