Is there any unwritten rules regarding hardware?? Knobs should go on drawers, handles on cabinets or vice versa?? Or only a certain number/combination of them used??
There really aren't any rules. It looks better if all are of the same finish, although I've seen numerous examples of mixing painted with metal; otherwise... it's pretty much whatever appeals to you.
"Heavy" handles and large knobs are usually visually preferred for base doors and drawers. If you are looking towards the future, handles with crossbars or a wide loop which are designed so that your fingers can curl around are somewhat better than knobs because arthritis and injuries make knobs difficult to grasp and pull. I personally find "fingertip" handles awkward and uncomfortable.
Styles and number of handles used are usually associated with ease of opening: drawers 15" or less wide work satisfactorily with one handle/knob centered. Wider drawers will slide easier with two handles/knobs which are usually placed 25% of width from either end. If you purchase ready-made or custom, they often come pre-drilled for attaching the knobs at the right distance. One word of caution -- many upper/wall cabinets have the knob centralized vertically on the door edge -- be sure the shortest person can comfortably reach that handle!!