From what I understand, unlike humans, female BP's will ovulate (release eggs from the ovaries) only once for the entire breeding season. She will store sperm from mating in the weeks leading up to ovulation, then at the time of ovulation, she will release the eggs (follicles) from her ovaries where they meet the waiting sperm and get fertilized. I believe "gravid" implies that the female has ovulated.
So, they CAN become gravid from one lock. But as I said, it doesn't happen right when they are mating. It happens later, when the female ovulates and the eggs are fertilized. However, you want to pair them up often in hopes that she will store up lots of sperm and fertilize lots of eggs. I'm assuming the 5-6 locks is a rule of thumb if you are trying to ENSURE that a female is gravid.