Hi,
Thank you all for answers. I tried and tested the scene. I used İtoo with Octane Proxy. Apparently the problem is Vram. With 980 ti I have 6 gb of Vram. I reach the limit at approximately 13-14 million polys. It looks low ,if 8.5 gb can handle 40 millions. But after 15 millions it become very unstable, sometimes works some times fails to render or even crashes.
I must say that 15 millions is not much, just some grass areas with 5-6 mid quality trees. I rendered the scene in several parts, to avoid crashes. I will post it later, to give an idea about grass trees objects amount.
Is the polycount affected by displacement, is it counted also as geometry? In the end I also removed all displacements.