It first appeared in "All Good Things" in the false future created by Q, when two Klingon vessels destroyed the USS Pasteur before being given a pasting by the refitted Enterprise-D. These ships were not given class designations, and it's perfectly possible that they would have had their own class name since they were significantly different from the ship subsequently identified as a Negh'Var. We should also bear in mind that the "All Good Things" future was not a real one, but was rather a fantasy created by Q.

The first ship positively known to be a Negh'Var class vessel was the ship seen in "Way of the Warrior". Sisko identified it as the Negh'Var, and said it was the "new Klingon flagship". The ship appeared to be significantly larger than the Vor'Cha class vessels which accompanied it, and was armed with two HUGE disrupter cannon. From this I've guessed that the Negh'Vars main job it to batter down heavy shielding - which is exactly what she did at Deep Space Nine.

We next saw a ship that looked very like the Negh'Var in 'Shattered Mirror', but there was one crucial difference - this version of the Negh'Var looked to be about two or three miles long! I tend to class this ship as being a scale up of the Negh'Var class itself, in much the same way that the K'Vort battlecruiser is a scale-up of the B'Rel Bird of Prey. Lots of people call this ship a 'Voodien' class (don't know if I spelled that right), although where that comes from I don't know. It's quite possible that the ship was intended to be the same as the Negh'Var, and the special effects guys just messed it up - Trek is notorious for scaling ships poorly against one another.

So far as we know there is only one Negh'Var, but I've suggested that there are others on the way - it seemed logical, given the situation with the Dominion, that the Klingons would be turning warships out just as fast as they could.