The red wiggler (binomial name: eisenia fetida) is the world's most common composting worm.
As a member of the epigeic– Latin for “on the earth”– class of composting worms, red wigglers generally will not be found in soil. Rather, they thrive in and under leaf litter, manure, decomposing vegetation, and other organic matter. These worms thrive in rotting vegetation, compost, and manure. 
Once these worms eat decaying food they produce worm castings. Which then can be used to amend the soil , feed plants
The amount of red wigglers you buy should be solely dependent upon the surface area you have available for vermicomposting.
As a rule of thumb, you should buy 1/2 to 1lb of composting worms per square foot of vermicomposting surface area.