The original question asked how many earthworms there were per egg which is, of course, only one. Earthworms form cocoons with many eggs in them, which is likely what the asker was referring to.
Earthworms are members of the sub-class Oligochaeta which has over 10,000 species. Thus, a precise count is not possible as different species produces different numbers of young.
This site says one cocoon can contain between one and five individual worms.