The annoying answer is: enough. You need enough oil to coat the bottom of the pan in an even layer. It will also depend on how much fat is (or isn't) in the food, and what cooking method you're using. Sometimes I feel like the low/no fat movement has people conditioned to use as little oil as possible in their pans. The truth is you do might need to be a bit more generous with it — the oil, heated properly, is what will help things not stick!