Mice seem to have quite a good representation of the 3d environment around them and motor skills. I had one in my flat run off an jump through an approx 1 x 2 inch hole 6 inches off the ground and about 10 inches from where it jumped from. Humans would probably have a job with that and I've not seen a lizard say seem to have similar ability to know its way around.

I daresay I don't think animals actually need some number or neurons. There's probably just a trade off between more giving better results versus being heavier and more energy consuming.

Speed may be a factor - reptiles and mice live their lives at very different paces.