Altered Ego

X can be 0. Altered Ego won't enter with any additional +1/+1 counters, and it will just be a copy of the chosen creature.

If you copy a double-faced creature, Altered Ego will be a copy of the face that's up when Altered Ego enters. Because Altered Ego is not a double-faced card, it won't be able to transform. If an effect instructs you to return it to the battlefield transformed when it leaves the battlefield, it won't return and will remain in its new zone.

If the chosen creature has {X} in its mana cost, that X is considered to be 0. The value of X in Altered Ego's last ability will be whatever value was chosen for X while casting Altered Ego.

Altered Ego copies exactly what was printed on the original creature (unless that creature is copying something else or is a token; see below). It doesn't copy whether that creature is tapped or untapped, whether it has any counters on it or any Auras and Equipment attached to it, or any non-copy effects that have changed its power, toughness, types, color, or so on.

Any "enters" abilities of the copied creature will trigger when Altered Ego enters the battlefield. Any "as [this creature] enters" or "[this creature] enters with" abilities of the chosen creature will also work.

You can choose not to copy anything. In that case, Altered Ego enters as a 0/0 creature and is probably put into the graveyard immediately. It won't have +1/+1 counters placed on it by its ability.