Depends on it. A building model from the dead mod USkaarj for UT2k4 has 9566 tris. I think this is allready much for UT2k4, but if i play USkaarj mod with several buildings placed, all with such a tri count and a large map...the game still runs fine.
As mentioned in another thread, it seems to really depend on the end-user hardware, I think so.
My Specs are:
Intel Core 2 Dou 1.8 Ghz
ATI Radeon X1950 Pro
3 GB Memory
So, LLM_Xunami, I think you can create models up to around 10.000 tris. This is at least a thing, which will still work. You should consider to keep the tri count as low as possible, to ensure, that even people with low-end systems (which can run UT2k4 fine on highest settings or lower) can play your mod (if you are working on one) or can use this model in UT2k4.