It depends on the recommendation of the manufacturer.
If it says 20 ml, 10 doses it means 2 ml of the vaccine shall be given per animal.
Example
Pestiffa( Merial) is 2 ml per animal (they have 20 ml, 10 dose)
Aftopor(Merial) is 1 ml per animal (50 ml, 50 doses)
Farrowsure(pfizer) is 5 ml per animal(50 ml, 10 dose)
REspisure(pfiser) is 2 ml per animal (20 ml 10 dose)
So far i have heard of vaccine with the following dosage 0.5 ml, 1 ml, 2ml, and 10 ml but i have never encounter 3 ml.