Really, we only saw a tiny fraction of the 40 Starfleet starships that were assured to be there. And I don't mean (just) tiny fragments - I mean the fragments never added up to 40 ships. More like a dozen at best.
So quite possibly the battlefield featured a lot of shipwrecks that never floated into the camera view. There might have been up to a hundred Federation ships (since it is never confirmed how many took part in the battle, merely that exactly 39 perished), dozens of Klingon vessels, and perhaps a few Romulans or Tholians thrown in the mix.
Timo Saloniemi