Apparently, more than 50% of all singleton puppies die (source: the internet so take it for what it is worth:)). Many of these puppies die because they grow too big in uteral and the bitch has difficulty with whelping (many taken by C-section).

Are Singleton puppies healthy?

Though rare, these "singleton" puppies, as they're colloquially known, have unique care needs. Without the chance to interact with littermates during the first couple of months of their life, these puppies can develop a range of behavioral problems.

Is it bad to get a singleton puppy?

Because the singleton pup has no other litter-mates, he will strongly bond and attach to humans, but lack of socialization with other pups may lead to future problems when he must relate with other dogs. The pup will fail to learn the basics of social dog language and the important basics of bite inhibition.