According to IMDb, Mark Sheppard appears as Crowley in a total of 67 "Supernatural" episodes, though three are labeled as "uncredited." Considering that the series ran for over 320 episodes and 15 seasons, this may be fewer episodes than fans would originally assume, but he is still the character with the fifth-most appearances, right behind Jim Beaver as Bobby Singer. Overall, Crowley is always more of a come-and-go character, even in storylines that prominently feature him as the antagonist, so it's not entirely surprising.
Crowley's final episode is the Season 12 finale "All Along the Watchtower," which sees the character have one final face turn and sacrifice his own life in order to trap Lucifer (Mark Pellegrino) in an alternate universe. Again, not exactly a surprising end for the character in a show that routinely kills off fan-favorite heroes and villains in equal measure. Still, Sheppard's Crowley is arguably the only character to vacillate between hero and villain so often, and in ways that endeared fans instead of annoying them. This is likely what made Crowley such a popular character in the first place.
