Here is how Democrats can get 60 seats by January 2021.

2018- Democrats end up with 50 seats by losing ND and gaining AZ and NV.
Between 2018 to 2020- A Republican US Senator from a state with a Democratic Governor that is not up for re-election in 2020 resigns or dies unexpectedly.
1)Murkowski(R-AK) Begich-D
2)Rubio(R-FL) Gillum-D
3)Isakson(R-GA) Abrams-D
4)Grassley(R-IA) Hubbell-D
5)Moran(R-KS) Kelly-D
6)Kennedy(R-LA) Bel Edwards-D
7)Burr(R-NC) Cooper-D
8)Portman(R-OH) Cordray-D
9)Toomey(R-PA) Wolf-D
10)Thune(R-SD) Sutton-D
11)Johnson(R-WI) Evers-D
The Democratic Governor picks the replacement that has an excellent chance of winning the 2020 special Election.
2020- Democrats pick up
1)AK- Sullivan-R
2)AZ special
3)CO- Gardner-R
4)GA- Perdue-R
5)IA- Ernst-R
6)ME- Collins-R
7)MT- Daines-R/Bullock-D
8)NC- Tillis-R