Density is a trade-off. More cells means more accuracy and more time; fewer cells means faster, looser drawing that keeps your own hand in it.
A useful starting rule: aim for cells about the size of the smallest feature you care about. For a portrait, that is roughly the size of an eye — usually a 8×10 or 10×12 grid on a head-and-shoulders reference.
For gesture and figure studies, drop to 4×5. The grid is there to keep the pose from drifting, not to trace it.
For architecture and anything with straight repeated elements, go denser — 16 columns or more — because errors in windows and brick courses are instantly visible.
If you find yourself drawing three or four separate shapes inside one cell, the grid is too coarse. If most cells are empty background, it is too fine.