Assembled Ensemble will count itself as long as it’s on the battlefield and remains a Robot, so its power will usually be at least 1.

Artifact creatures that appear in the art of other cards qualify. For example, if Assembled Ensemble appeared in the art of another card, casting that spell would cause the Assembled Ensemble’s last ability to trigger.

Robot is a creature type. There may be creatures that appear to be robotic in nature that don’t have the Robot creature type. They won’t contribute to Assembled Ensemble’s power.

A spell that causes the last ability to trigger doesn’t have to be an artifact creature spell. It only needs to have an artifact creature in its art. It also doesn’t matter how many artifact creatures are in its art, as long as there is at least one.

Creatures that are clearly artificial in nature also qualify, even if they aren’t their own card. The artifact must be clearly animated in the art in question. For example, Dancing Scimitar is definitely an artifact creature, but that doesn’t mean every sword is an artifact creature. Most aren’t.