We're loathe to pin down an exact number for that until the story demands it. However, there are twelve Terran Fleet administrative commands, each one (in the post-Capella) era subdivided into one, two or more battlegroups - strategically mobile in a way the Fleet structure as a whole is not.
With the number of destroyers per battlegroup generally either one or two (though sometimes zero, and sometimes more), you can guess that the two dozen figure is probably in the ballpark (though perhaps on the low end) of the total number of Terran destroyers before the new ones started rolling out. There aren't many Raynors and Titans compared to Hecates yet.
As a point of addendum, the battle groups active in the Terran theater are drawn from multiple Fleet commands so that they can circulate their experience within the Fleet structure when they're rolled back from the front. With ~six destroyers generally running active flight ops in relation to the Sol theater and the 14th's ships off active duty, I'd guess in a non-official way that you've got something like a quarter to a third of the Fleet busy with the civil war.
