Though the average time of fertility is from 8 to 14 days, not all dogs are the same. One of my Basset females tends to be ready as early as the 5th days of her heat (five days from the time she first shows signs of bleeding and swelling). Until a female is fertile, she will refuse to allow the male to couple with her. Once my females are willing, I allow them to breed freely with the mate of choice. I currently have a pair who have been mating approximately 4 times per day. When her time of fertility has passed, she'll let the male know in no uncertain terms that the party is over. There are no negative effects to allowing a pair to breed at will and I have never heard of a female aborting as a result. That doesn't make a bit of sense.
In a free state with many males available, a female dog will mate many times; I have seen street female dogs mating 10-12 times a day
One time mate a female pregnant
You mate a female on the 9th day of heat 11 day and the 13 day of heat