On the "mechanic" parts there are main game and 2 add-ons - Mask of the Betrayer and Storm of Zehir. All three tells different stories and two expansions adds more functionality.
But it was promoted as "game system", just like NWN1, so it's possible to make infinite amount of modules. Only handful of them was made "official" though - but it still too many. And I cannot check _how_ many, as it seems it stopped to sells as soon as "NWN Enhanced Edition" was released.
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