
There may be many eggs fertilised, but a lot depends on the implantation stage and how well they space out and how good the environment in the uterus is, and good placentas develop.
Also your more likely to maximise conception by mating for as long as the bitch will stand, thereby getting past the usual reason for failure to get a litter mistiming the mating.
Bitches especially that have had previous litters may stand for prolonged periods, so without blood testing you only have the stud dogs and bitches behaviour to go on, which in these circumstances could be well out, if you rely on one mating.
Mating on alternate days maximises the quality of the studs output.