It's not a good idea to let them 'do it as much as they want.' In addition to risking injury, the males sperm count is only going to be able to stretch so far. They key would be to accurately time the controlled breedings by accurately determining when the female is the most fertile. Progesterone testing to determine that is very beneficial. 2-3 ties within a span of 2-4 days should be more than sufficient.
Curious as to why you'd be asking this now as it appears your girl is already expecting. Just future reference, I assume? Given that her breeding was successful, how did
you time her breedings? (# of tries, days, etc) What worked for you?
Some opinions differ on the latter part of your question, regarding the premature puppy aspect. But from everything I have ever learned, that is one issue you hope to eliminate by not spacing the ties out over a long period of time.