Many modules allow for more than one henchman. And, in fact, I believe that the Bioware expansion packs, Shadows of Undrentide and Hordes of the Underdark also allow more than one henchman. (I am not sure about SoU, but I know that HotU does.) If you have the Diamond edition, then you have those expansions already. If you buy NWN from GOG (which is Diamond edition, but already updated with the latest patch), then you will have them as well. I assume some of the premium modules allow for more than one henchmen, but I almost never play with more than one, so I can't be sure.
There are lots of excellent community-made modules (which are free, btw) that allow for more than one henchman. As an example, the Aielund Saga (first chapter
here) does. I don't know that there is any list of modules broken down by max henchmen, but the readme files will sometimes mention it. A nice module list is available here, in addition to those that have won other acclaim, such as the
NWN Vault's Hall of Fame list of modules and award winners from the
Academy of Modding Excellence.
(BTW, if some of the terms aren't famliar to you, a useful stop is always the
NWN Wiki.)