You definitely want to have a good amount of spares. Especially if it's a car with a lot of custom one off stuff. And like Joe mentioned, if you don't have a lot of spares, just bring extra cars to use for backup.
This is the amount of stuff I typically take to the track if I'm staying relatively close to home (within a 2 hr drive). Radios, batteries, chargers in the green tote. B2/B3/T3 parts in the blue tote. RC10 parts in the Worlds box. Extra tires in the cardboard box. If I'm going to a bigger event like Vonats or the vintage race in NC, it's more. If it fits in the car I want it to go along.