The Republicans will take 2-4 seats, depending on whether Obama gets re-elected. Given the huge amount of seats that the Democrats need to defend (23, compared to just 10 for the Republicans), the Democrats are almost certainly going to suffer a net loss even if Obama gets re-elected.
In terms of determining how the parties need to delegate resources, the number of seats (2:1) that Democrats will be defending seems important. What's more important is
which seats each party needs to defend, and in what type of environment. In 2010, the GOP had to defend almost as many seats (even seats with incumbents) as the Democrats did, but how many of those incumbents would have been deemed "vulnerable" in
any environment? Maybe one or two?
Point is, if the Democrats have to "defend" 20 seats, but only 2 or 3 appear to be vulnerable (e.g. unpopular with constituents, etc.), and Republicans have to "defend" 10 seats, but 4 or 5 are vulnerable, does it really matter how many seats each party is "defending"?