"J'ai des robes."
Translation:I have dresses.
In English, "have" changes to "has" when the subject is 3rd person singular (he, she, it, one, someone, a person, an animal, a thing).
In French, verbs change form or endings with almost all types of subjects (grammatical persons); so you have to learn verbs' conjugations:
The conjugation of the verb "avoir" in present is: j'ai, tu as, il/elle/on a, nous avons, vous avez, ils/elles ont.