All ambientCG assets are provided under the
Creative Commons CC0 1.0 Universal License.
This applies to the downloadable asset files and the "preview spheres" (material preview renders) shown on the site.
The Creative Commons CC0 license gives you near limitless freedom to use the materials, models and other assets:
"You can copy, modify, distribute and perform the work, even for commercial purposes, all without asking permission."
You can read the full text on the Creative Commons website.
You don't need to give credit but I would appreciate it.
Here is how you can do that.
If you wish to give credit you can do so using this text:
Created using <asset name> from ambientCG.com, licensed under the Creative Commons CC0 1.0 Universal License.
Alternatively you can also use this shorter version.
Created using assets from ambientCG.com (CC0).