Citadel Fleet

The Citadel Fleet is the main space defense force of the Citadel Council. Destiny Ascension is the flagship. The fleet consists of a mixed group of Turian, Salarian, and Asari ships, though the exact number of ships is unknown.

Saren, allied with the Reaper dreadnought Sovereign, launched a surprise attack against the Citadel. During the attack, the Destiny Ascension ordered Citadel control to close the stations arms, creating an impenetrable fortress. However, Saren had used the conduit to infiltrate the Citadel and shut down both the Citadel and the Mass Relay network, subjecting the fleet to the full force of the geth attack and preventing any reinforcements from arriving. Sovereign then breaks through the fleet and closes the Citadels arms behind it.

The Destiny Ascension managed to evacuate the Citadel council but was unable to fight its way through the hordes of geth ships. Commander Shepard eventually reactivates the Mass Relays, allowing a human fleet and the Normandy to come to what was left of the Citadel Fleets aid. It is then up to Shepard to decide whether to have the fleet save the council and the Destiny Ascension, or to let them die while they wait for the arms to reopen.

Aftermath
The fate of the fleet ultimately depends on your Paragon or Renegade choices...

If you choose to save the council, then it is known that at least the Destiny Ascension survives. It is not known how much of the rest of the fleet survived.

If you let the council die, then it is implied that almost the entire fleet is destroyed.