For generic characters, different art = different card. So Angry Mob, Elders of Jerusalem, Pharisees and Sadducees can get multiple in a standard T1 deck, since they have 3-4 different arts.
Some cards have an identifier on them that overrides the deckbuilding rules, like Legion, Angry Mob, or Locust from the Pit.
Other cards that band to themselves, like Absalom's Soldiers, are limited to 1 per deck in T1 since they don't have different art. Those are more intended for T2, where you can have up to 4 in a standard deck.
