On a side by side house that has two meters and two separate main panels. How many ground rods are required

If you don't have another grounding electrode, or just water mains, you need two rods, no matter how many meters there are

If you need ground rods (i.e. there's no other grounding electrode save for metal water service lines), then you only need two of them, no matter how many meters are present at the structure. This is set out most clearly in NEC 250.58:

250.58 Common Grounding Electrode. Where an ac system is connected to a grounding electrode in or at a building or structure, the same electrode shall be used to ground conductor enclosures and equipment in or on that building or structure. Where separate services, feeders, or branch circuits supply a building and are required to be connected to a grounding electrode(s), the same grounding electrode(s) shall be used.

Two or more grounding electrodes that are bonded together shall be considered as a single grounding electrode system in this sense.