both are correct in different ways, no one is going to kill you for putting more than one tag on a post. tags don't take up space. just do whatever (within reason)
Cat: this chost contains a cat, I'll probably use this even if there's multiple cats
Cats: this is a chost about cats, the subject, talking about them or owning them
CATS: ALL YOUR BASE ARE BELONG TO US. YOU HAVE NO CHANCE TO SURVIVE, MAKE YOUR TIME


