Campden is used for a couple of reasons- one is when the must is made, to kill wild yeast and bacteria. After that, it's used as an antioxidant and preservative. In order to acheive that, the goal is to keep 50 ppm of so2 in the wine. If you don't have a meter, the best guestimate is to use one campden tablet (crushed and dissolved) per gallon of wine at every other racking, and at bottling.
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