Types
There are many types of earthworm species, but Red Worms, or Red Wigglers, are considered best for farming. Red Tigers are also used and can be mixed with the Wigglers.
Number
Since earthworms only eat about their own weight each day, it's best to have at least a pound of them to efficiently process any fuel you give them. According to Three Trees Farm, a pound of earthworms contains anywhere from 800 to 1200 worms, from young to full grown adults.
Habitats
Plain dirt, sawdust, shredded paper and peat moss all make good homes if mixed with manure.
Diet
Earthworms will eat just about anything organic in nature, including kitchen garbage, paper, manure, grass and leaves and leftover food.
Considerations
Keep their soil bedding moist but not over-saturated with water. Be sure to check out the size of their home and readjust if necessary--the worms will double in population about every three months.