No dog of any breed should have puppies more than once a year at the very least. they should have at least one season's rest inbetween pregnancies so I would say no more than a litter every 18 months.
The kennel club will not let you register more than 1 litter a year from a bitch. This is why you get these puppy farms selling puppies without papers, because they are overbreeding their dogs to maximise their profits. Unfortunately this leads to weaker puppies and puts an incredible strain on the mother, leading her into poor health.
If you are considering breeding, please look into it carefully. We bought a CKC spaniel last year after being told she was spayed. she had pups 7 weeks after we got here (obviously pregnant before we bought her!) and we subsequently found out they'd been overbreeding her. she had a severe heart murmur and couldn't bring the litter that she had after coming to us. She had to have a caesarian and died about 6 months after having the pups because her heart never recovered from the strain of that last pregnancy. It broke our heart and we've now decided not to breed our second CKC as we couldn't bear the same thing to happen to her.