Apparently in Season 4 (https://en.wikipedia.org/wiki/Charlie%27s_Angels_season_4), there were 12 solo episodes, three of which belonged to Shelley Hack (https://www.reddit.com/r/television/comments/gh1ru6/how_come_shelley_hack_gets_such_a_bad_rep_for_her/). Then, there were 13 team episodes, one of which was a backdoor pilot (the "Toni's Boys" (https://www.metv.com/stories/there-was-almost-an-all-male-spinoff-of-charlies-angels) one), two of which were a two parter (https://charliesangels.fandom.com/wiki/One_Love...Two_Angels_(Part_2)) where the other two angels compete for the same man (https://www.reddit.com/r/CharliesAngels/comments/1s0pz3c/where_charlies_angels_really_went_wrong_in/). So all in all (https://www.datalounge.com/thread/36390993-original-%E2%80%98charlie%E2%80%99s-angels%E2%80%99-stars-reunite-for-50th-anniversary-event-in-la), Shelley Hack's character (https://charliesangels.fandom.com/wiki/Tiffany_Welles) in the 25-episode long season (https://drunktv.net/2020/05/01/charlies-angels-season-4-1979-1980-tv-series-review/), is barely present in like half of them.