I am playing on a Realm in normal mode and found a Nether fortress. The rates at which wither skeletons, skeletons, magma cubes, and blazes spawn is just chaotic. There would be clusters of 3 to 5 wither skeletons with blazes and just maybe 20 seconds after I kill them they respawn and I'm back the square one. I got to an area where I could see the surface of a large portion of the fortress and there were so many mobs all over the place that it was almost comical.

The world and its Nether were originally loaded in 1.14 but the fortress was loaded in 1.16 (there was nether gold ore in the area). I was playing on the full release.