How do you determine how many fighters/torpedoes your plan can support at any given time, and I am talking about the fighters and torpedoes that you actually have directly on your planet, not in your sector defenses. Anyone can help me determine the proper amount?

Your plaents can support whatever tech they have plus twenty worth. So if your ship has 300 torps, and your planet has 280, they can use the same amount. Still, more will always help you, so that you don't run out after being attacked one or two times.