Comparing 6 different prediction sites, all agree that Republicans will pick up at least 6 seats, with the most likely being Alaska, Arkansas, Iowa, Kentucky, Louisiana and South Dakota. The only exception is Real Clear Politics, which is making the most conservative estimates (scientifically conservative, not politically conservative) and declaring many of the polling so close that they are still within the margin of error, so keeping them as "Toss-ups" (although they have published a "no toss-up map" that agrees with the other 5 prediction sites, that the GOP will pick up 6 seats). Of the other differences between the sites, one is Kansas, where Politico and Washington Post are calling likely Republican, whereas 538 and Princeton are calling "leans Independent". Both of the latter along with the Washington Post say Georgia is slightly leaning Democrat, and both Colorado and Louisiana are leaning Republican, each of which the other 3 sites still call toss-ups. Sabato at the UVa Center for Politics is still counting Kansas as a toss-up.
However, that number "6" is contingent on a couple of things. First, it presumes that Republicans would be certain of keeping all of the seats they currently control. But three of these seats are actually far closer than expected--Georgia, Kansas and Kentucky. In Kansas, Governor Brownback has made the Republican brand so toxic that the incumbent Republican senator may get kicked out of office, replaced by an independent who has not stated for whom he would vote for Senate leader, but he would not support either Reid or McConnell. So while McConnell's seat may end up being safe from challenger Lundergan-Grimes, unless Republicans vote in a new Senate leader other than McConnell, Orman might refuse to caucus with them, potentially giving Democrats a hail-Mary if the race is closer than expected. The second problem for Republicans is Georgia, where they lost their safe incumbent Saxby Chambliss, and now that race may turn into a runoff which 3 of the prediction sites are calling a toss-up, and the other 3 are saying leans slightly Democrat.
The second "6 seats to victory" contingency is that the close races will cut evenly between Democrats and Republicans. If the 6 states listed above all go for the Republicans, which all of the sites agree is likely, and they keep Georgia and Kansas, then they are safe. However, if either one of these states goes for the Democrats, then Republicans will need Colorado and/or Louisiana for the definitive win. Three prediction sites (Politico, Sabato and RCP) are not calling either of these races, leaving them as toss-ups as of Oct 26, while the other three sites (538, Princeton and WaPo) are calling them likely Republican (in the "no toss-ups" model, RCP calls both of these for Republicans). If Republicans bring in BOTH Colorado AND Louisiana, then they don't need Georgia or Kansas for the tie-breaker. But if three of these 4 states (GA, KS, CO, LA) go for Democrats, then the situation gets complicated.
First, if the Senate decision comes down to one seat, then Georgia or Louisiana may have to break the tie, and neither of those races may be determined by this election due to the nature of their system. For example, in Georgia, if none of the candidates break 50%, it requires a runoff which would be held in mid-January. Louisiana's potential runoff would be in early December. Second, if 3 of these borderline states go to Democrats, then the Senate would be tied, meaning Vice President Biden would be needed to break any tie votes.
So what all of this means, if that the Senate goes the way that all of these 6 sites predict, based on polling as of October 26, then control of the Senate in 2015-2016 will likely be Republican. The spoilers are Georgia, Kansas, Colorado and Louisiana. Republicans only need 2 of these for the win. But if Democrats get 3, Biden will be needed for a tie-breaker, and if they get all 4, then Democrats remain in control of the senate.