One per car as for onroad your battery is taped in. The ir is also lowered through the day the more you charge. The variable ir readings are because the bullet connectors arn't tight enough. Discharge-charge 40 amps every run to lower ir for mains.

I replace my race batts when I see a difference when running.. id has never been a factor I have some race packs that have a very high ir. over 16mohm and still get great runs. I only have a pack and 1 replacement per class. I run 4 when I can most a 3 class limits..

For onroad there is a significant difference especially in spec classes.