[Edi]

Yes, it's hard limit for descriptions added by mod. You can go over the limit via multiple mods or just one megamod that has that many descriptions. Any more than that and they start getting overwritten. It's a limit that seems to be set in stone, but it was discovered through trial and error and probably has something to do with array sizes reserved for that kind of things.

I have not tested it myself, I've compiled those bug reports just on the basis of what Sombre, Ich and the others have said, since they can generally be trusted to report things in depth and accurately.