For most normal gravity beers, one dry yeast packet will do just fine. However, if you are brewing beer with a high starting gravity (above 1.060) you may need more than one packet.
Jamil Zainasheff has created a great yeast calculator
on his Mr Malty website that will tell you how many grams of dry yeast you need to add to your beer based on your starting gravity and the age of your yeast packets. As yeast ages in the package, it loses viability and you need to add more to compensate for that in addition to adjusting based on your starting gravity.
I have always had good luck adding just one package of dry yeast for beers with an initial gravity below 1.060.
The Homebrew Nerd