Can a reminiscence hold many items? Example sentence:
A reminiscence I'll never forget are the days I started noticing her.
We don't use reminiscence in the sense you appear to intend.
A reminiscence is an act of remembering (or perhaps even more often an act of narrating), not the object of the act, the thing or matter which is remembered.
It is acceptable to speak of the object as a memory, but properly it is a memory of [the object].
A memory I will always cherish is that of the days . . .
But in the presence of forget, either reminiscence or memory is pretty much piling on redundancy. It's much simpler and more natural to say
I will never forget the days . . .