The realistic range for Republicans in the Senate in the 2018 election is from a net loss of 2 seats (lose Arizona and Nevada, gain nothing) to a net gain of 10 (hold all R seats and gain all 10 viable seats).
The Democrats have to defend 25 states and the R's only 8. Six R's are safe (MS, NE, TN, TX, UT, WY) and 15 of the 25 Rats are safe.
The likeliest pickups are IN, MO, MT and ND, though the Missouri GOP is an absolute joke, led by an idiot RINO governor, and they are shooting themselves in the foot at every turn. ND is hardly a lock for a pickup and IN/MT are no slam dunks either.
The "tossups" (to put the best possible spin on it) are FL, MI, OH, PA, WI, WV -- all Trump states in 2016, some very narrowly. WV should have been a somewhat easy gain but not if Mr. Sore Loser Jailbird gets on the ballot as a third-party spoiler to save Manchin.
Wisconsin is a tossup-in-name-only at the moment (D incumbent clearly favored, but as long as we're dreaming here...). All of the other tossups are uphill battles against incumbents. Dimwitted, overly-liberal, do-nothing incumbents (Casey, Brown & Stabenow in particular) to be sure, but they got elected before and they can damn sure do it again. Whoever is charge of the NRSC better be ready to fight to get the best nominees wherever primaries have not yet happened, and then spend big time in the general to get out the vote.
Thank you. You are the only realistic poster I have seen in a long long time here on FR. We need more like you - informed and actually understands politics.