Is “J'en ai” the best way to say “I have some”?
What's the difference between “J'en ai” and, “j'ai un peu”, or “j'ai quelque chose”?
J'en ai is a good translation of "I have some".
J'ai un peu alone is incorrect, you should state what it is about, e.g.:
J'ai un peu d'argent -> I have some money.
J'ai quelque chose is correct French but while j'en ai is used when your party knows exactly what you are talking about, J'ai quelque chose is used to state you have something but without telling precisely what it is, i.e. "I have something".