There's a shortage in Napa recently, and it's becoming newsworthy:
According to Crystal Britto, owner of Concierge of the Valley, getting
a cab in Napa on the weekends can be difficult. With only three main
taxi companies in town, riders have fewer choices for such
transportation, she said.
Juliette Christensen, head concierge at Andaz Napa, said that during
busy times of the year it can be hard to get a cab in Napa. “Sometimes
it can be 30- to 45-minute wait,” she said.
The key 'problem' is the lack of a large enough community requirement for it, so they have to plan around seasonal variation:
The need for taxis in Napa Valley is seasonal, he said. “Right now, we
run four to five cabs per shift instead of all 12. During tourist
season we run 10 cabs minimum,” Bulger said.
so if a bunch of people need cabs at the same time, especially during off-season, you may have problems getting one, or at least a decent wait.
From TripAdvisor - some people wondered about getting taxis to take you around all the wineries in a day. The estimate there was $50-$80 per cab ride - presumably as they're not all that close together.