I haven't been able to find this information elsewhere, but I'm working on a Dwemer ruin, and basically I need to know just how much stuff I can cram into it before the CS and/or game goes "YOU BROKED MEH!!!111!!" and crashes.

I always try to keep it under 1000. Which is usually very easy to do.
I have noticed crashes and/or a major loss in FPS if you use too many Lights or NPCs... so you want to limit them as well. (I can't remember the exact number, but too many Lights can be especially crashy)

I'm not sure if Statics count towards this number or not, but the Overflow Loot Bag will appear after 1024 items are in the cell (as soon as the Player interacts with the cell) so you want to avoid anyone accidentally creating that.