Spells that were cast before Breeches entered the battlefield count. If Breeches was the first spell you cast this turn, the next spell you cast this turn is your second spell.

Breeches's triggered ability triggers without requiring a target. It sets up two delayed triggered abilities: one that triggers when you win the flip, and one that triggers when you lose the flip. Only one of these abilities will trigger, based on the result of the flip. If you lost the flip, you'll choose a target for that delayed triggered ability at that time. Players can respond to the delayed triggered ability knowing its target and how much damage will be dealt, as normal.

If you don't sacrifice an artifact, you won't flip a coin, and neither delayed triggered ability will trigger.

If you win the flip, the copy created by Breeches's delayed triggered ability will have the same targets as the spell it's copying unless you choose new ones. You may change any number of the targets, including all of them or none of them. The new targets must be legal.

If the spell that's copied is modal (that is, it says "Choose one —" or the like), the copy will have the same mode or modes. You can't choose different ones.

If the spell that's copied has an X whose value was determined as it was cast, the copy will have the same value of X.

You can't choose to pay any additional costs for the copy. However, effects based on any additional costs that were paid for the original spell are copied as though those same costs were paid for the copy too.

The copy made by Breeches's delayed triggered ability is created on the stack, so it's not "cast."