100 AMP Service - Breaker Box - obviously residential. Must you use a grounding rod? If so, how many? Inside or outside the foundation? What gauge (copper wire)?

The grounding rod can be anywhere close to the panel. I've seen rods pounded through a hole in concrete below the panel in a basement installation.

You will need 2 copper ground rods 8 foot long each and drive them in the outside earth until flush with the ground. They must be 6 feet apart from each other. You have to run a #6 gauge wire from the panel box ground bar to the new ground rods outside and clamp the wire to each rod

Not to add any confusion, but I need to add some detail.

If the home is served by a metal water pipe that has at least 10 feet of pipe in earth, it SHALL be the primary grounding electrode, and there SHALL be at least one ground rod, with the #6 copper wire coming from the neutral/equipment ground bar in the panel connecting to the water line, no farther than 5 feet from the point of entrance into the home and the ground rod.

If the home is served by a plastic water line, then the two ground rods method is acceptable.

Many times, homes served by plastic water lines have metal water lines inside the home, and if so, the water lines shall be Bonded to the neutral/equipment ground bar by a #8 copper wire.

Minor note, when two ground rods are driven, they must be AT LEAST six feet apart, they can be farther way from each other, but not closer. As mentioned, the top of the rods must not be above ground, all 8 feet must be in earth, and the ground clamp connector must be rated for direct burial.