Of the 18 competitive Senate races (this number doesn't include Vitter, Burr, or the seat in West Virginia), Republicans would need to win 16 to secure a majority, and certainly logic suggests that the odds of achieving this would be long in any remotely normal year. But the operative term is "in a normal year," which this is most certainly not.
The Senate editor of The Cook Political Report, Jennifer Duffy, notes that the toss-up races don't always break evenly. She points to the Democratic wave year of 2006, when the party won 89 percent of the nine races that The Cook Political Report rated as toss-ups before the election. In 2008, Democrats won 78 percent of the toss-up races, while in 2004, a good year for Republicans, the GOP won 89 percent of the most competitive races. In other words, these wave elections produce a cascading effect in which the close races often break disproportionately toward the wave. The exception was 1982, when the anti-Republican wave that hit the House missed the Senate as the closest races broke in the GOP's favor.
Ace has some more musings on "wave" dynamics.