It's not like there is a magic number. Like if I were to say, if your little one reaches 12 lbs, she'll be ok. That's not how it works. You have to take into consideration her body structure, can she safely carry and whelp a litter? Does she come from a line of free whelpers? (mom, grandma, aunts and sisters, can they whelp on their own with little to no complications) Is she healthy? And no, a regular vet check is not enough to check for health. There are specific tests that must be done.
What kind of stud will you use? What does his family lines bring to the mix? Is there any incidence of any genetic diseases in both the dam and sires lines?
So you see, it isn't so easy if you want to do it right for her, her puppies, and future puppy parents of the pups.
__________________ Littlest JakJak 
We miss you Kaji 
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