How many trucks can line up in one dock outside a speakeasy?
Well, I'm going to risk making the obvious answer.  There is nothing in the rules which limits the number of trucks allowed in each dock... probably because there is no limit to the number of trucks allowed in each dock.

Looking at this example paragraph in the rulebook (p11):


  b. If the demand has not been fulfilled, the Speakeasy must continue
  buying Crates from the Truck(s) at the Minority Dock, again paying the
  Wholesale Purchase Price to the Crate supplier.  The Trucks are lined
  up in Muscle card order, so Mobsters with higher Muscle cards are more
  likely to sell their Crates.  Any Crates in excess of demand go unsold
  and are lost.


Truck(s) clearly implies that there can be multiple trucks at the Minority
Dock.  All the stuff about lining trucks up in Muscle card order suggests to
me, albeit more vaguely, that there can be quite a few trucks there.  If there were only 2 trucks allowed at the Minority Dock, you'd expect some kind of phrasing such as "the Mobster with more Muscle cards", rather than "Mobsters with higher Muscle cards", wouldn't you?