The number of worms you buy is dependent upon the surface area of your bin. In general, you want to start with 1/2 to 1 lb of worms per square foot. For red wigglers, this amounts to 500 to 1000 worms per square foot.
We recommend starting an Urban Worm Bag with 1-2 lbs of either Red Wigglers or European Nightcrawlers.