When using a grid for drawing subject matter, it is really dependent on how complicated the image is that you are trying to draw. For instance, if you use more blocks, it is going to help you focus on smaller details much more, however, it will also require more time to complete your drawing. On the other hand, if you use fewer blocks, this will make the blocks larger, which means you will have to focus on capturing more of the image within that single block. Consider how complicated your drawing is and your intention. If you would like to capture an image as closely comparable as possible, then using more blocks will allow you to capture smaller details more accurately.