If they were churning out litters, then I would worry. But then it doesn't matter if they are churning out one breed, or 7....
This. It's quality, not quantity. A lot of people I consider good breeders (they health test, prove their dogs in conformation, perfomance, keep them in the house, etc.) have 2 or 3 breeds they work with.
Often it's a show breeder who's made friends with other breeders, and gotten interested in another breed that has different things to offer than their "main" breed. For example, it's not uncommon for people with big hounds to keep small dogs like paps or whippets around. And since they're into showing, they get nice prospects from their show friends and they show the 2nd breed too. Often they'll co-own, and sometimes the "secondary" breed will have a litter under the guidance and mentorship of the original breeder.
I hate the idea of being limited to one thing. I love borzois and will always have some around, but there are also things I like to do that aren't a borzoi's strongest trait. Tracking for one. Just because I like tracking doesn't mean I have to adopt a rescue to track with and never compete or breed it because I'm already involved in one breed.
Some day I will have a good show/working prospect who was mainly bred for nose work. Maybe a spinone, maybe a beezer, or a treeing walker, or even a fila, but if I prove it's worthy of being bred and my mentors in that breed agree, it might get bred. Would that make me a bad breeder?