One of Falcon's traits is called Patriotic Duo; it reads as follows.

Patriotic Duo: When a friendly character named Captain America is adjacent to Falcon, they both modify their attack values by +1 if not already modified by this effect.

It's obvious what happens if Falcon is next to a single Captain America named Captain America, whether it's Steve or Bucky or Isaiah under the mask.

What happens if he's adjacent to multiple Caps?

Take the following, with the four characters in a sideways "T" shape, with Falcon at the middle. All characters are adjacent, something like the following imitation diagram.

Steve = S, Bucky = B, Isaiah = I, Falcon = F
S
FI
B

What happens? My BEST guess is that all four would get +1, and only +1, to attack, but is this correct?

You are correct. Falcon cannot get more than +1 due to the "if not already modified by this effect" clause.

Right. So it would also work for each instance of "Captain America" around him--that is, each Cap would also get +1?

I see no reason why it would not work that way.

Thought so! Just wanted to make sure. Thank you!