If the copied artifact has in its mana cost, X is 0.

As the enters-the battlefield ability resolves, first you scry 2. Then each opponent in turn order chooses whether or not to scry 1. Those who do (likely all of them) look at the top card of their library at the same time, then they decide in turn order where their card goes. Each opponent will know the choices of previous players in turn order before making their own choices.

If the copied artifact is copying something else, then The Mycosynth Gardens becomes a copy of whatever that artifact copied.

The Mycosynth Gardens copies exactly what was printed on the original artifact and nothing else (unless that permanent is copying something else; see below). It doesn't copy whether that artifact is tapped or untapped, whether it has any counters on it or Auras and Equipment attached to it, and so on.