And a Task Force seems to consist of mere dozens of ships - which TNG "Redemption" already establishes as far too low a number to perform offensive operations. So both designations probably have their place in the Starfleet terminological hierarchy, which in this sense appears to closely resemble the good old US Navy one.
Actually on Call to Arms, the Federation/Klingon task force was exceptionally larger than 12 ships. So the reality is it doesn't seem like there's a limit to number of ships for a task force designation. It appears that the number of ships are variable per fleet and task force. What's significant is a task force appears to be composed of a mixture of ships from different fleets or different races. And a fleet is only federation ships. In my opinion, you could call a fleet 5 ships or more, and the fleets 1-10 are heard of more because they were the ones of consequence. Meaning they were large enough and deployed at offensive fronts.