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.
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
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.
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.
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.