We are working on the new Interactive Map system to remove all warnings which will be available in the upcoming days.
Are you on Mac or Windows? Is it a 32 bit or 64 bit system?
As 8k resolution for terrain heightmaps is kinda heavy, the machine needs to be in 64 bit. Does Unity freeze before generating terrains in the scene and stops immediately?
Test with lower heightmap resolutions and see if the problem persists. Also lower the “Pixel Error Quality” in the New Terrain Settings for smoother preview in the scene.
FYI, there is no performance impact between the number of tiles. The total data resolution is the factor.
As for the case, I have generated a scene exactly with your settings with no problems at all as you see in the following picture.
Centered at Hollywood Sign, 80km2, 8×8 (64) terrain tiles, 1024 resolution for each terrain tile’s heightmap & image (8192 Heightmap/Texture resolution) ending up with approx. 10 meter per pixel resolution for both heightmap & imagery.