What is maximum/recomended ref count for interior cell? And what exactly happens if i exceed it? Does interior do not load at all, or does it kills fps or something else?

I've never read of anyone hitting the "threshold", but obviously as the numbers increase performance will decline. It's really not as simple as a number; it will depend of course on what type of object is being placed. Lights, animated models, actors, creatures, particle effects, high-poly models (custom high res textures as well) will have a greater impact on performance than tile sets and low-poly statics of the same number. Another factor in performance was how many game units are being rendered in the interior. A space of 15,000 units with 300 objects will perform with a deficit as opposed to an interior half that with the same objects on the longest axis.

Back in the day, as a rule, I limited myself to a cap of 300-400 objects, and a face count of 70,000 to 80,000 before placing actors, creatures, lights, etc. Granted, we have ten years of hardware advancement since that benchmark, so feel free to test the limits. One caveat; the game engine will "clean up" loose clutter (any object the player can inventory) in an interior and generate a loot bag. IIRC I think the limit on that was 1024.

Unfortunately I can't be helpful, but there is a treshold after which the game will crash, no matter how powerful the PC is. The fact that the limit for interior is far lesser than that of exterior goes without saying. Sorry I can't offer any specific numbers. :/