And technically, Okuu is Soku's penultimate boss, with Suwako being final boss (see Arcade mode, and her lack of Story mode).
One thing no one has brought up is the number of named background characters.
Both Hakurei Shrine and Human Village are popular hangout spots, so it's reasonable for them to have the largest number of characters - 9 named characters, and many extras. But even the Palanquin Ship stage still has 7 named characters.
Since EoSD, there has been about 70 named characters who have appeared in gameplay. Add Daiyousei, Koakuma, Tokiko, Rinnosuke, Akyuu, Kasen, Kosuzu and the Lunar Capital characters, then you've got about 80 characters, a few of which probably won't appear in this game. Supposing each stage has on average 1 playable character + 5 background characters, that results in at most 13 stages + playable characters.
Of course, the number of named characters in a stage background can be highly variable. But given the "popularity contest" theme, it's reasonable to think that most stages should have a few cameo characters, except for maybe a special Earthbound-style "final battle in space" "you need a prayer" "the whole Gensokyo is watching" kind of stage.