I have 10 SDs in my 180, and they're not crowded at all. I have different types, though, and some are smaller than others... so I guess you'd have to figure out what the max size is for the type you're buying. They're pretty close schoolers, but also spread out from time to time.
The only thing I'd advise with these guys is to use as much natural light as possible, rather than a strong tank light -- they're skittish, and the strip light doesn't agree with them... ours is usually off, but the tank receives a good amount of sunlight, so it's no biggie for us, but might be a deal-breaker for you.
The only thing I'd advise with these guys is to use as much natural light as possible, rather than a strong tank light -- they're skittish, and the strip light doesn't agree with them... ours is usually off, but the tank receives a good amount of sunlight, so it's no biggie for us, but might be a deal-breaker for you.