I’ve tested this, using a default terrain and exporting with 99458 polygones as 3ds. When I open it in Wings3D, I see the slices you report. I get 5 slices with 21,846 polys, 33,041 edges and 11,197 vertices for 4 of the 5 slices. One slice is smaller.
I’ve exported the same terrain with the same settings as Wavefront obj. Importing it in Wings3D I see only one object with 99459 polys, 149,633 edges and 50,176 vertices.
This slicing up and grouping is probably due to the 3ds format or how it is implemented in Bryce. I’ve never noticed this because I usually export the terrain as image - 16-bit greyscale.
If you can use the Wavefront obj format in whatever application you import the terrain, I suggest that you use obj. It has the benefit of saving either the mesh, the material applied to it, or both.