It's a wide range from what I've seen with the group I race with. For me it's a spare battery for each vehicle. Basic tool kit. A spare spur/pinion gear or part. Couple of the guys have enuff to completely rebuild vehicles. Setup stations, scales, charger, one even had a tire warmer for his F1.
I'm only 5 races in. I use time between to just get as much practice as I can.