The first number in each line is how many episodes were filmed, the second is where the show ranked for the season, and the third is the average number of viewers in millions.
Supergirl's viewership was boosted in Season 1 by its running on CBS. In Season 2 it moved to The CW.
Arrow
Season 1 23 130 3.68
Season 2 23 128 3.28
Season 3 23 135 3.52
Season 4 23 145 2.90
Season 5 23 147 2.21
Season 6 23 181 1.76
The Flash
Season 1 23 118 4.62
Season 2 23 112 4.25
Season 3 23 120 3.50
Season 4 23 151 3.04
Gotham
Season 1 22 68 7.56
Season 2 22 89 5.37
Season 3 22 100 4.52
Season 4 22 129 3.69
Supergirl
Season 1 20 39 9.81
Season 2 22 129 3.12
Season 3 23 154 2.82
Legends of Tomorrow
Season 1 16 135 3.16
Season 2 17 141 2.57
Season 3 18 170 2.24
This data has not yet been posted for Black Lightning and Krypton.