Depends on a couple of things.
1) The OG of the beer you plan on making
2) If its an ale or lager
3) How old the yeast is
4) If you plan on pitching the appropriate amount of yeast
If you're going to be lager brewing and want to pitch the appropriate amount of yeast, you will to be making almost a 4 liter starter for 1 packet of liquid yeast. ~1.050 OG if the yeast is 3-4 weeks old.
Checkout this yeast pitching calculator which helps a lot http://www.mrmalty.com/calc/calc.html