Hmm. There are limits to what Lightwave can handle, but in reality your PC would probably choke before you actually hit those limits.
I'd imagine that the whole thing was rendered in several separate segments and layers, then composited together.
FYI, this is taken from the appendix in the manual:
LightWave Limits- unlimited points and polygons per object
- 1,023 points per polygon
- 4,096 bones per object
- 32,000 cameras per scene (see note)
- 32,000 objects per scene (see note)
- 32,000 lights per scene (see note)
- 65,536 object layers
- unlimited images
- unlimited surfaces
Note:
Because of memory considerations, the available number of cameras, objects and lights per scene is set at 100, 1000 and 1000 respectively in the lw3.cfg file. The values may be increased - within the limits above - but not decreased below 100; however, this will require more memory, so keep the new values as small as possible. Do not edit the cfg file while Lightwave is running.