The official Orbiter recommendation is 20,000 polygons, but many add-ons exceed this easily and without problems.
The hard limit of Orbiter is 250,000 polygons per scene(DirectX 7 API), more and you experience strong fps drops.
My personal recommendation are 45,000 polygons as hard limit for the whole add-on at a time and 35,000 as design goal (at the same time - you can have completely easily more, but you should never show more).