How many seasons does the Attack on Titan anime have?

How to get close to this number

Apply the anime scheduling grid. A long adaptation runs multiple cours spread across years, and four seasons is typical for a manga adaptation that runs to completion rather than stopping partway. The four-season broadcast calendar from this site's companion question is what shapes the count.

The split final season is the complication: the fourth was divided into several parts released across three years, which means the number of seasons and the number of release blocks diverge sharply. Some sources count the parts separately, which is why published season counts for this series disagree.

The completion is the notable part. Most long-running anime stop before the manga does and leave the story unfinished, because the adaptation runs out of source material or commercial support. This one adapted its manga to the end, which is rarer than it sounds and is why no further seasons are planned rather than merely unannounced.