2000 AD- At any time during the game, you may declare an opposing team or
character as your target. Once the character or team is chosen, it may not be changed. All 2000 AD team members increase their attack value by 1 when attacking the chosen team or character.

I understand declaring an opposing character part, but I don't understand "declare an opposing team". Does this mean choosing a opposing team "symbol"? Like if my opponent was playing all DangerGirl figs my 200AD would be bonuses agains all of them? I assume it doesn't mean opposing team as in opponents whole team in multi-player games?

If I had 2 or more 2000AD figs do they stack? or do I only get one choice of opponent for all my figs?

So when the ability references a team, it is referring to the team affiliation symbol that appears on a figure's base.

A player can only declare the 2000AD ability once in the game. And it is the player who declares the target, keep in mind; that player gets exactly one declaration, and it applies to all of his 2000AD figures.