I have been told that ie, if by accident, a collection is spilled then you can collect same day again in a few hours. What to say as a normal schedule - not more than every other day for optimal results. Why do you ask?
Wild dogs may breed several times in a day, at least, as will a male dog running with his domestic dog pack, according to old time breeders. So, yeah, they can be bred many times a day perhaps, but with probable diminishing returns after a while.
The repro vets definitely collect a small-breed male more than once in a few hours if necessary in order to get enough of a good collection to freeze in a straw. Not so much a problem in most Labs! One good Lab collection might be split between at least two bitches. However, a girl's owner may not be happy if that is done without checking the collection. The bitch owner puts a lot into choosing a stud and trying to have a litter. and deserves to get good treatment. Not sure want motivates your question.
It all depends. It might be OK with some potent studs, and a bad idea with a less potent stud. If you are all that curious, take your boy to a repro vet, have him collected and get a professional opinion.
The real answer is as many times as he can without hurting himself. Ejaculate will always contain sperm. However, if you collect (or breed) too often, the male can get sore... (No detail needed I'm sure!) If you need to breed two bitches at the same time, you can split the collection in half and there will still be millions of sperm for each girl.
It takes an average of 5 days of daily breedings or collections to reduce the dog's semen volume to his "daily sperm output", or what his testicles are actually producing on a daily basis.
As soon as the dog is ready to go for it again. Some would go again once a day, and some are ready in just 30 minutes.
They can breed several times a day for 5-7 days with little effect on fertility. If kept on breeding the sperm count suffers and immature sperms start to show reducing fertility.