Larry J. Sabato suggests that control of the US Senate could come down to eight races:

Let’s assume that, at the dawn of the 113th Congress in 2013, all 67 sitting senators not up for reelection this year — 30 Democrats, 37 Republicans — return to serve next year (no departures for the Cabinet, the Court or the Great Beyond). Next, let’s also assume that the 16 races we currently favor Democrats to win go to the Blue column, and the nine races where Republicans are favored go to the Red column. (See our full chart below.) Note that we have long flipped Nebraska and North Dakota from Democratic control to Republican control; former Democratic Sen. Bob Kerrey’s return to Nebraska hasn’t moved us a bit. Note also, as we said above, we are assuming that Maine elects King, who in effect becomes an Independent Democrat akin to Connecticut’s Joe Lieberman or Vermont’s Bernie Sanders. Further, our analysis has Democrats holding seats that are actually or potentially competitive, such as Ohio, Michigan and Hawaii. Finally, we presume that Democrats don’t score surprising upsets in places like Arizona and Indiana.
With those assumptions in place, the Senate is tied exactly 46 to 46, with eight toss up races to decide whether Senate Majority Leader Harry Reid (D) continues to lead the chamber, or whether Minority Leader Mitch McConnell (R) takes over.