I think you really won't hit that limit with car racing. At big flying events there are many hundreds of trasnmitters being used at the same time.
At least on my racetrack I doubt that I will ever see more than 100..50...40 active at one time.

probably not a problem, but if you get any missed lapped counts, then I would try to get some distance away any other electronics and mount it as close to the loop as possible... so if the loop is buried, then low and flat on the chassis toward the front of the car, if the loop is above the track, then mount the PT high, many Rx boxes have mounts above them so the PT is shielded outside the Rx box.

I've seen some cases of missed laps caused by a worn servo connector that needed to be replaced on an older PT.

100 is still a tiny number compared to infinity, but yes you're unlikely to get collisions. However at my track two guys consistently interfere with each other. They're both running 2.4GHz radios, and if they're both on then both cars go haywire. One is a Futaba 3PV and I'm not sure what the other guy has. The 3PV guy has a second 3PV which doesn't have the issue. They've tried rebinding, swapping receivers etc. Nobody can figure it out.

Seen this happen with other brands of radios (Spektrum, Radiopost, FlySky)... I always thought the common denominator was direct sequencing and the fix was to swap out the Tx... one guy would sell his Tx and then buy a replacement (same brand/model) and that seemed to fix the issue... some of the older designs would use a fixed set of frequencies and sometimes those fixed frequencies would overlap. We also saw some of the newer systems were smart enough to scan the range of "in use" frequencies and dynamically select a fresh set of frequencies for their hopping pattern. Some guys with the older radio that didn't dynamically hop would intentionally leave their radios turned on all day long just to keep their set of fixed frequencies reserved and minimize the chance of glitches.