"We only have one dog."
Translation:우리는 개 한 마리만 있습니다.
Just to tell people here, I am a native speaker, and this isn't the natural form what we say. It is right, but it is not natural. We say it like this.
우리는 [something] [a number that you have] [unit]밖에 없습니다. It would be like this in English: We don't have that many dogs except one. Awkward, isn't it? Well, just like the grammar and sentences out there. Not tellin' to change it, but just sayin' for people who wanna be more Korean-ish.
That's not something achieved by simply saying a sentence differently, but by learning more words and constructs and tons of practice speaking (and speaking to other people!)
Those are two ways to say the same thing.
I'd say yours might emphasize a little bit more there is no other dog but that one.