This is one of the fundamental questions in the theory of groups known as the width problemwidth problem.
One of the best (general) references devoted to this problem is the book Words: Notes on Verbal Width in Groups by Dan Segal.
Yet another good reference is the paper Width questions for finite simple groups by Martin Liebeck.
A well-known conjecture due to Babai states that there must exists a constant $c$ such that $N(S,G)<(\log|G|)^c$ for all non-abelian finite simple groups $G$ and generating sets $S$.
L. Babai and A. Seress, On the diameter of permutation groups, European J. Combin. 13 (1992), 231–243.