|Number of seats on a bench|
|Used on these elements|
|Status: in use|
|Tools for this tag|
This key is commonly used in a somewhat difficult to verify manner, as a guess by the mapper as to how many people could comfortably fit on the bench at once. This makes it less useful than it might be if it was limited to being tagged on benches with individual seats, e.g. those with armrests in between each seat. Nonetheless, that isn't the current state of the map.
Other features, such as amenity=restaurant,tourism=alpine_hut, use the key capacity=* to state the maximum number of persons who can fit in the feature. For benches, seats=* is by far the most popular key for this purpose. (Taginfo shows 280,000 uses for bench seats compared to 2600 uses for bench capacity.)