" . . . With the safe arrival of the convoy PQ1 to the Vegan system, our escort duties are now complete, and I've set course for home."
That's from "The Sound of Her Voice"[DSN6]. That implies that at maximum, there could be 6,760 convoys by such a numbering scheme (26 x 26 x 10), with that one being the 2,721st, more likely than not.
Of course, the idea of a convoy going toward Vega (i.e. awfully close to Earth) doesn't make that much sense.
But just for kicks, we know that a convoy in "Waltz"[DSN6] was carrying 30,000 troops. If each convoy carried that many, then the total Federation ground force could number 81,630,000 troops.
Note that this is just a number . . . not a maximum, minimum, or recommended estimate.