I agree with your interpretation, no matter the number of allies, you only get +2 to your Shield bonus to AC.
Benefit: When you and an adjacent ally are each using a shield, your shield bonus to Armor Class increases by 2.
I read an here as at least one. Nothing in the feat explicitly calls out that it might scale.
Feats that have scaling bonuses generally have a much more specific wording. For example looking at (first find on Internet) Teamwork Benefits:
Your team coordinates its attacks with great precision when surrounding a foe, gaining benefits beyond those of standard flanking maneuvers.
Benefit: Your benefits increase based on the number of "flanking pairs" attack the same foe. If you have 2 pairs of flankers on a single foe, all 3 of you gain the +4 to hit instead of the standard +2. If you have 3 or more flankers the bonus increases to +6.
Another example, from the War Master's Charge maneuver (Tome of Battle):
As part of this maneuver, you charge an opponent. [...] For each ally who charges, counting yourself, your charge attack and those of your allies are made with a cumulative +2 bonus (in addition to the normal bonus provided by charging). [...]
This for each is fairly typical in my experience.
Out of RAW or RAI, I would imagine that the bonus could scale pretty high, with finely packed creatures... at the same time though, Shield Bonuses to AC do not count against Touch AC by default and do not protect against magical attack, and a dense pack of creatures is a juicy target for an area attack.