Is there a reason that "mult" is not accepted here? "Eu am auzit mult despre tine"?
That's just how you say it in Romanian. Maybe it's easier if you think of the English sentence as "I heard a lot of things about you!".
This is a common short form of "am auzit multe povești ". You will find a similar case with "am văzut multe la viața mea" meaning "I saw many things in my life " - "am văzut multe lucruri "