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.

The creature that's copied isn't targeted by Polymorphous Rush. You choose that creature as the spell resolves. It can be one of the targeted creatures. A creature can become a copy of itself this way, though this usually won't have any noticeable effect.

If the creatures copy a creature that's copying a creature, they will become whatever the chosen creature is copying.

If all of the spell's targets are illegal when the spell tries to resolve, it won't resolve and none of its effects will happen. If one or more of its targets are legal when it tries to resolve, the spell will resolve and affect only those legal targets. It will have no effect on any illegal targets.

The target creatures copy the printed values of the chosen creature, plus any copy effects that have been applied to it. They won't copy counters on that creature or effects that have changed the creature's power, toughness, types, color, or so on.

The mana cost and mana value of strive spells don't change no matter how many targets they have. Strive abilities affect only what you pay.

If such a 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).

Effects that have already applied to the target creatures will continue to apply to them. For example, if Giant Growth had given one of those creatures +3/+3 earlier in the turn, then Polymorphous Rush makes it a copy of Pensive Minotaur (a 2/3 creature), it will become a 5/6 Pensive Minotaur.

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

This effect can cause the targets to stop being creatures. For example, if they each become a copy of a land that has become a creature, the effect that turned that land into a creature won't be copied. The targets will become noncreature lands.