
Unles a blood test is taken that pinpoints ovulation the best chances of sucess are to mate the bitch every other day for as long as she will stand (usually over a periond of about 6 days so no more than 3 matings). the reason for this is to hopefully cover all yiour bases to hopefuylly have semen ready for when there are fertile eggs.
Some bitches ovulate early in their season and some can be very late even after the normally accepted 21 days,a dn most failure to conceive is down to incorrect timing.
A dogs sperm should remain viable for at least four days, though some will still be viable even after a week). A bitches eggs are immature and take around 48 hours to mature after ovulation, so the ideal day to mate is two days after ovulation so that fresh sperm is available when the eggs are mature. You should theoretically get a fertile mating by mating from around two days prior to ovulation and 4 days post ovulation (as the eggs remain viable for only another 48 hours after maturity).
Of course the closer you mate to the days when the bitches eggs are ripe (ands bitches ovulate over a day or so) then the better the chances of a larger litter, so here mating more than once will increase your chances.
I ahve in fact had my largest litters all from single matings, and had my smallest to a maiden dog who was allowed to mate over a five day period with an older bitch, her it was probably the bitches age, and starting mating quite early that affected this.
My Champion bitch who isn't keen on the act of procreation was mated 4 days after ovulation (so just at the stage when eggs would have been dying off) and produced a smallish littter of four (she herself was one of a litter of 9 from her mothers first litter conceived on one3 mating, when two days earliuer she wasn't quite ready, even though she stood).