Looking at Ballotpedia’s page on 2018 Senate races here, they have a chart of competitive races.
According to them, of the 13 competitive races, only 2 have incumbent Republicans. For Democrats to take the Senate, they will have to win both of those (so, 100% of competitive races with incumbent Republicans). However, if Republicans take 1 of the competitive Senate seats they don’t currently have, the Republicans will still control the Senate (due to tie-breaking of Vice-President Pence).
So, Dems have to take 100% of the competitive seats controlled by Reps, and lose 0% of their own. Nice terrain.
Behind this is the fact that 26 of the Senate seats out of 34 up for election in the 2018 cycle are held by Dems or Independents who typically vote with the Dems (King, Sanders).
Countering this, you could argue more than 2 seats with incumbent Reps are competitive. For example, 270towin here lists 3 seats instead of 2 as competitive with incumbent Reps (adding Tennessee), because if any of Sabato, Inside Election, and Cook call it a toss-up they include it in that column. Also, which states are competitive can change – few would have predicted a close race in Alabama’s Dec. 2017 election early in 2017.
With those caveats, though, it looks like a very tough cycle for Dems in terms of taking control of the Senate.