It’s a major question on Wall Street and one that’s likely to come up in the November midterm elections, but so far the number of companies abandoning the U.S. for low-tax countries hasn't posed a serious threat to the country's finances.

And experts are not sure just how many more will exit, even though the Obama administration raised the alarm in July about U.S.-based companies leaving for cheaper shores, calling for “economic patriotism” from those businesses.

So-called tax "inversions" — the tax maneuver in which corporations merge with foreign businesses to claim residence in a low-tax jurisdiction — will cost the U.S. Treasury only about $2 billion in lost revenue annually, if left unchecked.

That number comes from the Joint Committee for Taxation, which prepares revenue estimates of tax legislation for Congress. The JCT projected that a Democrat-sponsored bill to end inversions would save $20 billion over 10 years.

That’s not a lot, says Kyle Pomerleau, an economist at the nonprofit Tax Foundation.

Pomerleau notes that over a 10-year period the corporate tax code as a whole will raise about $4.5 trillion. He adds that 76 corporations have undergone or considered inversions in the past 30 years, compared with the 5,000 publicly traded corporations and 1.6 million C-corporations in the U.S. overall. “So both on the company level and on the revenue side it’s really not that significant,” Pomerleau says.

Nevertheless, some analysts have suggested that there's a bigger risk. The competitive advantages of inversions — and the threat of legislation to cut them off — might provoke a rush to the exits for a sizable chunk of corporate America.

Edward Kleinbard, a former JCT director, told the Washington Post last week that the JCT’s estimate “didn’t fully reflect the sharknado of inversions that is about to happen.”

The criticism that President Obama and Treasury Secretary Jack Lew have directed at companies considering inversions suggests that they are also afraid of that scenario. But it’s just one of many possibilities.

“How can you know the answer? I don’t see how you could know the answer to this,” said Jane Gravelle, an author of a Congressional Research Service study of inversions cited by the Democrats who wrote the anti-inversion legislation.

An aide for Rep. Sander Levin, D-Mich., the sponsor of the bill in the House, said that the JCT’s $20 billion figure was “the most credible estimate that we've seen,” adding that “it’s hard to say how many are currently in the works, as it seems that more and more companies are looking for ways to go about it without calling it an inversion.”

There are a few factors that limit the number of inversions.

One is that very few companies are “institutionally set up for inversion ... a very small fraction of the corporate tax base,” says Patrick Driessen, a tax analyst for Bloomberg Government who previously worked for the JCT and the Treasury. Companies like Walgreens, which recently decided against an inversion, could face a costly customer backlash if they sought out another tax jurisdiction.

Another factor is that, following a 2004 law signed by former President George W. Bush to curb corporate expatriations, U.S. corporations must find a company in a favorable tax jurisdiction at least a fifth their own size to merge with — not an easy undertaking.

And lastly, corporations would risk instigating serious political blowback if they left the country en masse — especially during an election season. “If too many companies attempt to invert, let's say, for example, 50 big companies by the end [of] the year, that could provoke a legislative or administrative response,” Driessen explained.

The Democrats’ legislation, which currently is unlikely to gain necessary Republican support, could gain the votes needed to stop inversions.

Or the Treasury Department could be pressured into changing tax rules to penalize companies that leave U.S. jurisdiction. That could stretch the agency’s authority and fray international tax enforcement cooperation, but it is a possibility Lew has said he is exploring.