Personally I would just pitch one smack pack - if it's fresh.
Beersmith uses a calculation to figure the optimum pitch rate for your batch..you can also look at www.mrmalty.com
and use the calculator there. Fresh smack packs and vials have about 100 billion cells. This is perfect for a normal strength, 5 gallon batch. If you are going with higher gravity beer, you'd want to use more yeast.
To get the cell count up you could use a starter. Boil some DME, add a pinch of yeast nutrient, cool and pitch your yeast. Shake it around everytime you walk by for 12-24 hours before brewing - or use a stir plate.
For most all of my beers I make 1 liter or 2 liter starters with 100g DME per 1000ml water and set it on a stirplate for about 18 hours before pitching into my batch. But before I got my flasks and stirplate I just used single vials or smack packs of yeast into all of my 5g batches - with good results.