Largely depends on room size and content. A small room with medium level listening habitats could probably be served by two sb12nsd. Spirited listening in a medium to large space imo, requires a ported sub/s.
Also, multiple seats can benefit from smoother response, but there’s no guarantee that just putting a second sub where it fits will do that. Also, if your main concern is a single seat, get the best sub you can afford, and spend the time to integrate it. Normally I’d say start saving for the second one right away, but I don’t know your use case yet.