Yeah, mbwa is singular and plural. Most animal names belong to the so-called N-class and are identical in singular and plural. Iften, something else in the sentence helps you out.
anakimbia. = The dog is running.
wanakimbia. = The dogs are running.
You could also say:
mpenda mbwa. = Emilian loves the dog.
wapenda mbwa. = Emilian loves the dogs.
Adding the object prefix to the verb makes it more likely to be interpreted as definite though, so the sentence with the simple anapenda is a bit better for a general statement.