please explain when you need to declare which team ability a wildcard is using. can you do it during another player's turn?

A Wildcard can change TAs anytime it would be applicable to do so, but BEFORE a die roll is made.

For example, Wildcards can take the FF or Suicide Squad TA before the die roll for Earthquake or when an opponent is about to (probably) KO friendly figures that may activate these TAs.

Or Wildcard TAs can become JSA/Defenders when an attack is declared against them. Or Bat Enemy/SinSyn when attacking.

thanks...just wanted to clarify.

However, to clarify, it's my understanding that you can't switch TAs during an opponent's single action.

For example, this would not be allowed:
My Wild Card TA figure is targeted for an attack. I declare I'm copying Skrull TA; my opponent succeeds on the roll, and he can make the attack. I now declare Defenders TA to gain the defense of my adjacent Defender teammate; the attack hits. I now declare I'm copying Mystics TA to give a click of damage to my attacker.

In this case, I would be able to declare the first TA (Skrulls), but nothing after that. Correct?

That is correct, but because you can only copy one TA per action.

Note that you can copy a TA in the middle of an opponent's action before the dice are rolled, though. So your opponent declares a Charge and starts moving on your character. You have not copied a TA yet for this action. The Charge character then stops adjacent to your wild card and announces it is going to attack. You copy Defenders and raise your defense. That is allowed.

(Also note the Mystics was copied too late in your example because it was after the die roll.)

Correct. Declare once per ACTION.

Here's another example:

Amazo uses HSS option 2 to make a ranged attack on a Wildcard. I declare Bat Ally. That action is now illegal, so Amazo gets to pick a different action. He uses HSS Option 2 to make a close combat attack against the same wildcard. Since this is a different action, I now get to declare Mystics.

This IS allowed.

Actually, the rule is that you cannot use more that 1 TA in any single action, but you have the right idea.

HSS Option 2 requires a power action that allows close combat attacks. You cannot make a ranged combat attack with it.

(If Amazo was making a HSS Option 1 action and did it, you would be stuck with one TA for the HSS Option 1 action...so if Amazo originally tried a ranged combat attack and you copied Batman Ally, you could not then copy Mystics when Amazo tried a close combat attack because it is the same HSS Option 1 action. HSS Option 1, Charge, RS, and Pounce all tie you into anything you do prior to the attack unless your opponent is nice and allows you to take it back.)

Sorry for any confusion my post might have caused since I was answering WHEN it was declared, not how often.

Although one TA is allowed to be used in a single action, we really need to stop saying things like "Once per action" since Wildcards can be declared outside of actions...something becoming more and more important to understand with feats and BFCs.