The mana value of a strive spell doesn't change no matter how many targets it has.

If this spell is copied and the effect that copies the spell allows a player to choose new targets for the copy, the number of targets can't be changed. The player may change any number of the targets, including all of them or none of them. If, for one of the targets, the player can't choose a new legal target, then it remains unchanged (even if the current target is illegal).

If a spell or ability allows you to cast a strive spell without paying its mana cost, you must pay the additional cost for any targets beyond the first.

Any target opponents that are no longer legal targets by the time Call the Coppercoats resolves won't have their creatures counted when determining how many tokens you create.

You choose how many targets each spell with a strive ability has and what those targets are as you cast it. It's legal to cast such a spell with no targets, although this is rarely a good idea. You can't choose the same target more than once for a single strive spell.