Unless a Republican President has <40% approval, 2018 will result in an increased number of republican senators. The only plausible democratic pickup is NV (AZ isn't going atlas red in a midterm absent an absolute democratic tsunami), ND/IN will almost certainly flip, MO/MT will start with a definite republican advantage, if Manchin retires WV will almost certainly flip (if Manchin runs for reelection he should be okay), and OH/WI/VA/PA/MI/FL/MN (if Klobuchar retires)/NM could all flip in the right situation/climate. Remember, midterm turnout skews republican naturally, so even a republican president at 44/53 approval (or thereabouts) wouldn't be toxic, and republicans would still net at least 1 senate seat.
Only times a party has maid gains in congress in the last 100 years has been 1934 and 2002
I need maid gains so my house will be cleaned faster.