The ability of an Ivory Guardians will affect itself as well as all other creatures named Ivory Guardians (not just the ones you control).

Each ability applies a maximum of once, even if an opponent controls multiple nontoken red permanents or multiple opponents each control a nontoken red permanent.

The abilities from multiple Ivory Guardians are cumulative. For example, if you control two different Ivory Guardians and an opponent controls a nontoken red permanent, each of your Ivory Guardians will be 5/5.