The contest requests that you create something in a specific style proposed.
Check the "Visual Style" paragraph.
This means that while there's no mention of a limit in the polycount, you got to make sure the result is optimized for VR rendering. After all, everyone's assets will be fused into a scene by the guys at Mozilla for the VR demo thing. If you push forward a model that has 500K poly and it's just a small scene with a creature, it won't be "useful" for them.
So, from what has been asked, it has to be low-mid poly depending on the details displayed while the style is respected. If you need more polygons for a good reason, then use more polygons. If you only need those polygons to put details that are too precise for the style, you don't need more polygons.
Those are examples given:
This tower is 22K https://sketchfab.com/models/f2671e1d29474f7d848fcfaeffe1ac01
Its relief is detailed, while its texture is simple.
This town is 68.5k https://sketchfab.com/models/97e0ec56f2164bfab6874417a5470cdf
It has way more stuff than the single tower, but the level of relief isn't as high.
Yet both display the same visual style : Low number & bright colors with low amount of details.