First of all, I always assumed Bonus=Benefit in the context of solo tactics. Anything else didn't even occur to me.
Regarding your question, the FAQ offers a precedent for Benefit = Bonus in the context of Outflank's secondary ability.
Benefit: [...] In addition, whenever you score a critical hit against the flanked creature, it provokes an attack of opportunity from your ally.
An attack of opportunity is most definitely no Bonus in the sense of the rules. However, the FAQ states, with regard to exactly this part:
Inquisitor: If an inquisitor uses Solo Tactics (Advanced Player's Guide, page 40) with the Outflank feat (APG, page 165), does the enemy provoke attacks of opportunity when hit with a critical hit?
Yes, but only when the inquisitors allies score a critical hit against a foe that they both flank. In this case, the enemy provokes an attack of opportunity from the inquisitor. The reverse is not true, since her allies can only gain bonuses from teamwork feats if they themselves possess them.
So for the purpose of Solo Tactics, "getting an AoO" is considered a Bonus, however "granting your ally an AoO" is not, which to be honest, is a little odd, since the Benefit of the feat clearly describes the latter.
I think the general rule with Solo Tactics can be expressed as
"Anytime it would be good for you if one of your allies had the same Teamwork feat as you, you can pretend he has it."
This is compatible with the FAQ ruling and may or may not be RAI (We might never find out). Some specific Examples as to what this reading means:
- Outflank: "If your flanking buddy ally lands a critical hit,
if he had Outflank, you could make an AoO."
- Coordinated Reposition: "If your ally makes a 5 foot step,
if he had Coordinated Reposition, you could also make one as an immediate action"
- Escape Route: "If your ally threatens a square,
if he had Escape Route, you could walk right through there without provoking an AoO."
Per Crawford, the order is important.
Jeremy Crawford reversed his original ruling on this topic (found here). This reversed ruling was formalized in the 2019 Sage Advice column here.
This ties the causality of the option with the order of events. If you must do Y to qualify for doing X, then you must do Y first. You may not do X, and then qualify to do X by doing Y afterwords.
In this specific example, that means you must take the Attack Action before using your bonus action to shove someone 5ft.
Best Answer
I agree with your interpretation, no matter the number of allies, you only get +2 to your Shield bonus to AC.
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:
Another example, from the War Master's Charge maneuver (Tome of Battle):
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.