What is the first hint that we get in the Matrix series that Neo wasn't the first and only 'One'?
My gut instinct is in the Chateau scene, when The Merovingian says these two lines:
Your predecessors had much more respect
...and
Mark my words boy, mark them well; I have survived your predecessors, and I will survive you!
The first movie details there being at least one prior "The One" who set "the first of us" free. Morpheus explains this to Neo shortly after the explanation of the real world -- the battery scene and Neo vomiting.
MORPHEUS: When the Matrix was first built there was a man born inside that had the ability to change what he wanted, to remake the Matrix as he saw fit. It was this man that freed the first of us and taught us the secret of the war; control the Matrix and you control the future.
However, as for there being multiple prior 'The Ones' I believe you may be correct.
