References
From Episode 1 p22, this is the allocation I assume you are referring to:
As an effect of the way in which the adventurers have impressed the dragon council, it now falls to them to allocate the dragons that have pledged their service to the factions.
From Episode 6 p59:
Five ancient dragons make up the council charged with deciding their metallic kin’s response to the cult. Though they are not necessarily the wisest or most powerful specimens of dragonkind, all are well respected. Any decision made by these dragon councilors will be accepted by the metallic dragons as a whole.
and
To secure the cooperation of the metallic dragons as a whole, the party must conclude this episode with at least three dragon councilors having a friendly attitude and with none unfriendly
and p61:
When the characters have reached a satisfactory agreement with the dragons,
Not directly related but useful as guidance, from Episode 9 p86
Chromatic Dragons. The exact number of chromatic dragons present at the Well of Dragons is up to you, and could range from a few dozen to a hundred or more.
and p87
Metallic Dragons. The metallic dragons will be outnumbered by their chromatic cousins. However, they cooperate with allies better than the evil dragons do, giving them a greater impact on the battlefield.
Discussion
Like a lot of things in this module, DM discretion is involved.
The criteria for getting the support of all metallic dragons is clear. It seems to be implied that you can get partial support as well. So having got support, full or partial, how many dragons is that?
The information in Episode 9 gives you guidance for the total number of metallic dragons available for the final battle. Even if you operate at the lower end of the range, the number of dragons available at that point is ... adequate.
Remember, the dragons are preparing for the final battle themselves and they have things to do, people to eat, sorry meet, etc. and the PCs have to negotiate an agreement with the metallic dragons. Surely the number of dragons provided for immediate deployment would be a key part of that agreement?
The Council Scorecard also needs to be considered. If the PCs get no dragons then they are at a disadvantage in the final battle but the suffer neither advantage nor disadvantage with any of the factions. If they make concessions to get the aid of the dragons (and they will probably have to) then they hurt their rep with most factions. Giving a faction support can repair this (partially in some cases) and not giving a dragon can hurt them further. The dragons are therefore a bit of a poisoned chalice; gaining their support may cost the PCs the support of one or more of the other factions.
So, do what any good DM does in those circumstances ... cheat. Give the PCs enough dragons so that their choices matter.
This adventure takes a lot of forethought.
As the DM it falls on you to decide how much you want to either adapt your world to the players and how much you want to allow the players to change the world.
For example, you may have already decided that Tiamat is not ever going to rise in your world. You may either "redo" or otherwise give the PCs another chance at an asset they have failed to acquire. For example (no spoilers here), the PCs could need to save a village from an attack by a band of lizardmen. As a reward (unknown to them), they will be given the Amulet of Gygax, a key asset to advancing the story. Due to poor luck, and poor decisions, the lizardmen win and the PCs retreat. Rather than simply saying, "Well, you think you should try again, right?" or something along those lines, you could say, "That village is destroyed; as you lick your wounds, the lizardmen appear to be sending scouting parties down to the next village."
If you've decided that the PCs can affect the world, you might decide that the final battle is inevitable, but the PCs' success isn't. At that point, the campaign becomes one of surviving in occupied territory (and maybe plotting an overthrow).
The three items you ask about really boil down to a question (I think) of "How can I make this story fun for the players without railroading them?"
The biggest piece of guidance I can think of on that is to talk to your players about where they think they are in the game: "So Bob and I are preparing for the return of Lolth so when Tiamat shows up we'll have an ally."
Remember that this is a collaborative effort.
Best Answer
21 x 2 hour sessions
Well, that's how long it took us anyway. These were all online voice only sessions which tend to keep you more focused than FtF and some may have gone up to 3 hours when it wasn't a school night for my son. 4 players plus the DM (me).