Well, that 'fleet' was only a dozen starships, iirc. There were more on the way, but there were only a dozen starships able to respond immediately to the sudden appearance of a threat on Earth's doorstep. Which makes a certain degree of sense - barring the sudden appearance of a wormhole or similar phenomenon, any hostile force heading for Earth would have to cross half the Federation to get there, putting innumerable ships, starbases, and sensor posts between the borders and Earth, with sensor nets so thick that I doubt even a cloaked fleet could make it all the way to Earth without being detected. I think that Starfleet has taken up a policy of keeping many more ships close to home because of the Dominion War and the Borg Incursions, instead of spreading most of the fleet out exploring hither and yon, but I don't think the number of ships we saw in the immediate vicinity of Earth would have been much different at any other point in time - it wouldn't surprise me in the least to find a dozen starships in the general vicinity of the Sol system, passing through for repairs, refits, crew exchanges, resupply, and a variety of different tasks, capable of responding to an emergency on a moment's notice. I'd honestly expect there to be more, unless the "more on the way" included ships that were in the yards that had to take on crew or be hastily bolted up before they could join a fight.