To be fair, we have seen many seemingly central UFP locations devoid of starship resources. Even those starbases located at strategic hot spots, like the one where Picard went begging for blockade ships in "Redemption", only have a handful of vessels available at a given time, far too few for meaningful military action like the episode establishes. The modus operandi of Starfleet seems to be never to stockpile starships in tactically sufficient numbers, but rather to constantly deploy all vessels on "civilian" tasks, and then summon them back for military action as the need arises.
This would probably give a reaction time of a week for a serious border incursion anywhere away from the very core of the UFP... And quite possibly a reaction time of two years against the Dominion threat, as we saw in DS9!
Usually, only a single vessel is available for intercept, if the hero vessels we followed in TOS and TNG are any indication. And usually, that's enough, unless the enemy is really serious about it. Even if Starfleet can only send one ship at a time, the enemy will know about the retaliation that will come a few days later, and the even bigger one that can be arranged a few weeks later, let alone the really big can of whupass that is opened after Starfleet has had a few months to get its act together.
Timo Saloniemi