Once you choose to gain life using Ancient Cornucopia’s triggered ability, that ability won’t trigger again that turn.

Ancient Cornucopia’s triggered ability counts the number of colors the spell has (from one to five), not how many colored mana symbols there are in its mana cost or how many colors of mana you spent.

Ancient Cornucopia’s triggered ability resolves before the spell that caused it to trigger. The ability will resolve even if that spell is countered. If that spell is countered, use its last known information to determine what colors it was.