Hi took my girl to stud dog on day 12 tied for 15mins them a couple of days later but did not tie.i got the days mixed up which i thougth was day 11 turned out to be day12. So if it only takes once why do you need 2 matings so what are the chances of her taken thank you

The reason for two matings being better than one is to cover the fact that some bitches mayovulate early and some late, so wtih two matings you increase your chances that there will be live sperm ready to meet mature eggs at some point.

Say your bitch was mated day 12, but didn't ovulate until day 18, with one mating there is unlikely to be any live sperm left by day 20 when the eggs would have been fertilizable. 

Conversely if your bitch had ovulated on day 6 eggs would have been ripe day 8 and dead by day 10 or 11, so you would have mated too late.

There is a great variance between breed, family lines and individual bitches ast to when they ovulate, and are willing to be mated.

for best results it is a god idea to mate a bitch from the first time seh will stand, every other day until she won't, which is likely to be the space of around a week.

Two matings one aimed at not being too late for an early ovulater, and one that will strill catch a late one is the purpose of the usual 2 matings.

I have had a litter of 9 from one mating, also I have had an older bitch who was willing to stand rather early, and ended up with a small litter in spite of several matings, as I suspect that the majority of sperm was available too early.  Of course puppy numbers are more to do with the bitch and how many eggs are there, and still viable at time sperm reaches them.

"for best results it is a good idea to mate a bitch from the first time seh will stand, every other day until she won't, which is likely to be the space of around a week."

A situation where I wouldn't recommend doing this is if you have a bitch with a history of needing a planned c-section. With a week or so of breeding dates it can make it difficult to determine the proper day to do the section. Instead I'd recommend doing progesterone testing to determine ovulation.

I'm not sure I understand what is meant by 'a history of needing a planned c-section'. Surely in the vast majority of breeds a c-section is an emergency, not planned?

C-sections are usually planned with Brack breeds like the English Bulldog, Boston Terrier, French Bulldog and ect.... Even tho some may be able to whelp on their own but the majority is a c-section do to Bitches narrow pelvic and puppies big heads.