Well...23 of them are up for reelection in 20 months. Too bad we have to wait that long.
23 Dems, 8 Repubs and 2 Independents, plus Sessions (now held by appointed Luther Strange) seat are in the 2018 election.
State 2014 PVI Incumbent 2012 result as of Jan 13 2017 (Roth)
Arizona R+7. Jeff Flake (R) 49% R Likely R
California D+9 Dianne Feinstein (D) 63% D Safe D
Connecticut D+7 Chris Murphy (D) 55% D Safe D
Delaware D+8 Tom Carper (D) 66% D Safe D
Florida R+2 Bill Nelson (D) 55% D Tilt D
Hawaii D+20 Mazie Hirono (D) 63% D Safe D
Indiana R+5 Joe Donnelly (D) 50% D Tossup
Maine D+6 Angus King (I) 53% I Safe D
Maryland D+10 Ben Cardin (D) 55% D Safe D
Massachusetts D+10 Elizabeth Warren (D) 54% D Safe D
Michigan D+4 Debbie Stabenow (D) 59% D Safe D
Minnesota D+2 Amy Klobuchar (D) 65% D Safe D
Mississippi R+9 Roger Wicker (R) 57% R Safe R
Missouri R+5 Claire McCaskill (D) 55% D Tossup
Montana R+7 Jon Tester (D) 49% D Tilt D
Nebraska R+12 Deb Fisher (R) 56% R Safe R
Nevada D+2 Dean Heller (R) 46% R Lean R
New Jersey D+6 Bob Menendez (D) 59% D Safe D
New Mexico D+4 Martin Heinrich (D) 51% D Safe D
New York D+11 Kirsten Gillibrand (D) 72% D Safe D
North Dakota R+10 Heidi Heitkamp (D) 50% D Tossup
Ohio R+1 Sherrod Brown (D) 51% D Lean D
Pennsylvania D+1 Bob Casey (D) 54% D Lean D
Rhode Island D+11 Sheldon Whitehouse (D) 64% D Safe D
Tennessee R+12 Bob Corker (R) 65% R Safe R
Texas R+10 Ted Cruz (R) 57% R Safe R
Utah R+22 Orrin Hatch (R) 65% R Safe R
Vermont D+16 Bernie Sanders (I) 71% I Safe D
Virginia Even Tim Kaine (D) 53% D Likely D
Washington D+5 Maria Cantwell (D) 61% D Safe D
West Virginia R+13 Joe Manchin (D) 61% D Tossup
Wisconsin D+2 Tammy Baldwin (D) 51% D Tilt D
Wyoming R+22 John Barrasso (R) 76% R Safe R
From above possible 7 seat pickup by the Rs, making it a 59 seat majority. Needs a couple more to make it filibuster proof.