We can't equate the rules of grammar with the colloquial speech. Grammar is relatively fixed while speech can be any which way.
When we speak, we dont care about subject or verbs as long as the meaning is clear. That's why phrases are fine in colloquial speech. But English grammar dictates that a sentence must have a subject and a verb. It can have other aspects but to be called a complete sentence, it must have a subject and a verb. If it misses any of the two, it becomes a phrase. And by that rule, "how many cats" can never be a sentence because it lack a verb. Of course we use tons of phrases in our daily lives and this is just one of those.