how many bags are usually needed for one nest?

Because you are trapping foraging worker yellowjackets that leave the nest in search of food to feed the colony, and you are not directly treating a nest with this product, that is a difficult question to answer. It depends on how many yellowjacket nests are in your area and the overall yellowjacket population in your area. Yellowjackets usually nest underground or in enclosed spaces, so their nests can be hard to find and identify. In an average yard, a trap or two placed to divert yellowjackets from your outdoor living space should do the trick.