The more, the merrier!
a sentence? It doesn't seem to have a main verb, so I'm inclined to say no, but it certainly functions as a sentence in everyday speech.
I can think of three ways of analysing it:
- It's a sentence with no main verb. Is this even possible?
- It's a sentence with an implicit verb: something like The more we have, the better!
- It's not a sentence. But then, what is it?