Thickness required depends on your usage and expectations. Most folks won't need any more than 3/16", good for general underbrush and some light rocks. Not good for heavy rocks or resting the entire weight of the truck on. If you are seriously going to use them, I'd say 1/4"-3/8" would be sufficient. 1/2" if you want to get crazy...

The designs I've been working on use some 3/16" for some parts, all the way to 1/2" for real crucial areas. The trouble is making it modular enough to facilitate trail repairs, yet solid enough to make a difference...

Fasteddy's advice is occasionally sound...