I've heard so much about you
Said upon meeting someone that one has been told a lot about beforehand. Oh, Susan! Yes, hi, I've heard so much about you! A: "This is my husband, Rich." B: "Oh, it's so nice to meet you—I've heard so much about you."
I've heard so much about you.
a polite phrase said upon being introduced to someone you have heard about from a friend or the person's relatives. Bill: This is my cousin Kate. Bob: Hello, Kate. I've heard so much about you. Sue: Hello, Bill. I've heard so much about you. Bill: Hello. Glad to meet you.