as for how many sessions.. there is no real magic number just keep at it until he loses interest and hopefully you see an ovulation.
Some could breed 7-8 times with one male and only a ouple of times with another and have a clutch to the second male... Its all about when her body is ready to go through the cycle of follicles , ovulation ect and not how many times they lock (granted that the more locks the better though just for piece of mind)
Hope this helps