
I have always been told that two or three matings two days apart give the best chances of conception. Also most bitches will only stand for about 5-6 days during their season. From what I have read this is usually from around the LH surge (aprox 2 days before ovulation) to 4 days after ovulation.
This of course makes sense, as a healthy dogs sperm can live in the bitch for 5 to 7 days. So if mated at the start of the receptive phase the sperm should still be viable when the ova are mature (two days after ovulation). At the end of the receptive phase the ova will still be viable for about two days after maturation (4 days after ovulation).
Obviously the most fertile day to mate would be 2 days after ovulation.
My bitch who had blood tests to determine ovulation on day 10 and 11 showed signs of Ovulation being imminent, and on day 12 the test showed that she was ovulating. She was a maiden and refused to stand for her first chosen suitor (with whom I had tried her last season without success) and on day 16 (4days after ovulation) rather unenthusiastically stood for mating, but would not hear of it the next day.
Her mother for her first litter was mated on day 15, her second litter she was mated day 15 and 16. She was mated in between the two litteers to an old dog whose most recent litters had been two and a singleton. She stayed with him and was standing from day 11 to day 17, but no pups despite several matings, and the bitch that was in whelp to him at the time I used him also had just one pup.
So with a fertile young dog, and correct timing of mating, one mating is sufficient, but more matings help cover the bases in case there is variation in the actual fertile days as opposed to standing behaviour.
Bitches who havge been bred from previously will often stand for prolonged periods, and then the breeder could well mate the bitch much to early if relying on her receptiveness and just one mating.