Each block indicates an incumbent senator's actions going into the election.
Currently there are 12 seats on the Democrat side up to election in 2020. 2 seats have not announced their intention to run and are marked unknown. Here is the interesting piece I see here:
What district/state is D5? Is this NY?
On the Republican side we have 22 seats up for election.
3 Retiring 7 Unknowns 12 Running
https://en.wikipedia.org/wiki/2020_United_States_Senate_elections