MTG Treachery

The X in the unveiling triggered ability has the same value as the X paid in the Unveil ability. This is pretty easy to derive since there is no other source of X.
The token copies exactly what was printed on the original card and nothing else, with the listed exceptions (unless that permanent is itself copying something else; see below). It doesn’t copy whether that permanent is tapped or untapped, whether it has any counters on it or Auras attached to it, or any non-copy effects that have changed its power, toughness, types, color, and so on.
If the copied permanent is copying something else, then the token enters the battlefield as whatever that permanent copied, with the listed exceptions.

If the copied permanent has {X} in its mana cost, X is 0.
If the copied permanent is itself a token, the token created by The Mirror Maestra’s unveiling triggered ability copies the original characteristics of that token as stated by the effect that created it, with the listed exception.
Any enters-the-battlefield abilities of the copied permanent will trigger when the token enters the battlefield. Any "as [this permanent] enters the battlefield" or "[this permanent] enters the battlefield with" abilities of the permanent will also work.