
Surely it must be all 3
A Bitch will only produce a certain number of eggs
However a Dog will only produce a certain amount of spearm that is strong enough and healthy enough
Also it must be a lottery, as spearm can live a certain amount of days, and a bitch is only fertile for a certain number of days it must be possible for different eggs to be fertilised on different days, therefore also explaining the reason of most breeders wanting two or more matings.
However most of the time there will be more spearm then eggs. :)