The numbers are going to vary by breeder, and how the animal is marketed, and the breed.
Some breeds have been inbred to the point that they suffer from a high incidence of genetic defects. Some of these defects make it impossible for dog to live a long and healthy life. Most responsible breeders will euthanize these animals when they are identified. Another practice common in selective breeders is that they we sterilize the animals that are not of breeding stock, and sell those as pets. Responsible breeders will only breed enough pets to fill the demand. However some breeders we euthanize any excessive pet stock, to focus on their quality breeding stock, and others will sell this excess to pet stores without regard to how their animals with be treated after that.
Pet stores have are in business to make money and puppies sell really well... what does not sell well are adolescent dogs that have behavior problems because they have spent 3-6 months in a small display kennel. Many of the big chain pet stores return these adolescent dogs to their distributor who euthanizes the animals. I suspect this rate is not high, there is quite a bit of cost in having the dog in the store for 2 months to not have them sell, but it is also not zero. I seriously doubt that any of these chain pet stores will release the numbers because I am sure that for the entire chain the numbers are large enough to make pet advocates very upset.
The other issue is with wholesale live animal dealers. These include puppy mills, but also include the wholesale distributors that supply the chain(and some times even locally owned) pet stores with the live animals. These dealers do no want any puppies that are over 12 weeks old because they are harder to sell. Many of these wholesalers will take animals that are weaned earlier than is healthy so that they can get more puppies in the 6-8 week range to the pet stores because those sell the best. A number of those puppies pulled early will die in the first few days after weaning. If they do not get sent to a pet store before they get to 10 weeks the chances are very high that they will end up being euthanized. Again the actual numbers of this are going to be kept secret.