A sentence is incomplete if it lacks either a subject or a verb. The absence of at least one independent clause also makes a sentence incomplete.
The number of words in a sentence has nothing to do with preventing it from being a sentence fragment. "I agree" is a complete sentence because it has a subject ("I") and a verb ("agree"). "Stop!" Is a complete sentence because it has a subject (understood "you") and a verb ("stop").
A group of words can have a subject and a verb and still be an incomplete sentence, however, if no independent clause exists. "Because I have told you numerous times not to do that" has a subject ("I") and a verb ("have told"), but it is an incomplete sentence. The subordinating conjunction "because" makes this clause dependent. Removing the conjunction or adding an independent clause corrects the error. For example, "Because I have told you numerous times not to do that, I must ground you" is a complete sentence.