breeding frequency

Question
We have a female and a male that live here together.  We have decided to let them have puppies.  Again! last time was not planned, but hey got together once and had four beautiful puppies.  They all sold in one day.  And we had several requests from others to inform them if there was ever another litter.  
The question is how many times should they be permitted to mate during a single Heat?  Does this frequency have anything to do with the number of puppies they have?
Four is a rather small litter and eight is beginning to push it.  Is there a way to possibly control the number of pups that she has?
please excuse my ignorance.
Thanks
Ross

Answer
Hi Ross. Twice is enough. Any more than that and you have overkill. You end up with too many due dates, and may have a few premature pups as well. There is no way to control how many pups she has. Mom determines how many eggs are put out, dad determines the sex. If I can help with anything else, let me know.