The extra episode could be a sign that FX is happy with the ratings and viewership accumulated by Legion, although it’s also possible that the brass at FX simply realized that there was so much story to tell by the end of the season that the plot could be stretched out into an extra episode.
Legion Season 2 originally scored an order for ten episodes, but that order was bumped up to eleven back in May.
FX has not yet announced an episode count for the third season.
The first season ran for eight episodes, while the second season was a bit longer.