It will vary from batch to batch depending on the gravity. You can visit yeastcalc.com or http://www.mrmalty.com/calc/calc.html
and plug in your gravity, batch size, etc... to see how much yeast to use. You will be fine with one vial for anything under 1.060. If you're using dry, you can probably use half the satchet for a batch and save the rest in an airtight container for the next beer.