In 2020 Democrats will have to defend Doug Jones seat in Alabama which will be tough in an election year. Delaware, Illinois, Mass, NJ, NM, Oregon and RI should be easy retained seats for them. Michigan, Minnesota, NH, and Virginia will be the other competitive seats they will have to defend.
Republicans should retain Alaska, Arkansas, Georgia, Idaho, Kansas, Kentucky, Louisiana, Mississippi, Montana, Nebraska, Oklahoma, SC, SD, Tennessee, West Virginia and Wyoming. They should have tough races to retain in Arizona, Colorado, Iowa, Maine, and NC. Texas as well if Beto is the candidate.
So that's a total of 11 competitive races by my count ( I actually think 10 because I don't see Doug Jones retaining his seat in a presidential election year). Safe seats sees the Dems retain 7 and Reps retain 17, that's a margin of 10 with only 10 competitive races, so I don't see a path for the Democrats to get back control of the Senate in 2020.
Regardless of what we see as safe seats for each party, the GOP are defending 21 seats while the Democrats defend 11.
You only need to look at the 20/30 point swings in this years elections to realise that the GOP have got their work cut out.