Two is nice to have. I'm back to two cable providers plus DSL, but last year I had the choice of Comcast, or something slow, pricey and wireless. And that was in the Boston suburbs, not the mountains of Idaho or something.
Network infrastructure in the USA is startlingly primitive and monopolistic.
Many U.S. cities have historically granted right of way (to run cables) to one "phone" company and one "cable" company. In my area, we have AT&T and Comcast. It's up to the cities to allow more competitors, and many are unwilling to do that (or unable, if they have long-term agreements with the existing carriers).
It's a classic case of public policy lagging behind technology, with politicians having no concern for technological progress and free market principles, unwisely entering into long-term agreements with telecom companies.
Unless I'm terribly mistaken, one of the main reasons cities entered into agreements with cable companies, etc was that the cable companies wouldn't lay down cable unless they were granted a monopoly in the area.
(although thinking about it, whenever I hear about US isps it is always the same few companies (comcast comes to mind))