Not that many, and most of them are in Europe, which is having its own debt problems. Here's the list:
The question, of course, is what happens if the U.S. loses its AAA status? Bond yields will almost certainly rise, meaning as much as $100 billion more a year in interest on debt, according to Moderate think tank Third Way. From the WSJ:
Many others are downplaying the effect of a downgrade. AllianceBernstein wrote in a note that markets aren't particularly spooked right now, and there isn't much evidence that a downgrade brings anything new to the table. "If the only change between today and tomorrow is a credit-rating downgrade, history suggests that the market won't react severely," they write. Ratings agency Fitch also said the immediate impact on the market wouldn't be big. "Over the near-to-medium term, in a moderate downgrade scenario (e.g., to AA), U.S. Treasuries would likely retain their standing as the benchmark security that anchors global fixed-income markets, given their unparalleled liquidity, unique role in the financial system, strong credit profile, and lack of a viable alternative," Fitch said.
It is worth noting that the market is tumbling today, with the Dow down around 160 points with an hour of trading to go. News coverage so far this week has minimized the damage on Wall Street, but the truth is no one knows where this is going or what it might lead to. Many of the smarter commentators have kept their mouths shut, for good reason.