This is important (though not germane to numbers) - if I may make the suggestion that in the drivers meeting, or at least when assigning the starter position, it be mentioned that the starter should pay attention to the relationship of downed cones near the start and how long it takes the course worker to reset the cone before sending the next car.
I was fortunate that I wasn't run over after assuming the starter was paying attention to such things, and he wasn't.
As in boxing, "protect yourself at all times" when on the course.
As to numbers, I picked up a set with the intention of running in San Diego (never did make it out), but did use them when I went to Cal Club (once, then their "big course" was shuttered, too... ugh). And I picked up three, since a 3 digit number was a requirement for newbies. Gives a bit of flexibility to get a number not already taken, and, everybody likes numbers on their "race car" anyway!
However, the clubs are run by volunteers, and it's all extra admin work, so as cool as numbers are, it has to be easy to set up and easier still to maintain. CalClub has a sheet they carry over year to year, and they've somehow tied it to motorsport reg. $25 bucks for a number, so hopefully it not only covers the motorsport reg fee but also provides a little extra revenue for the club.
Haven't been to Phx in a while, but when I've been there in the past, it's usually functioned pretty well, no numbers needed.
My 2 cents, which in these inflationary times are worth less than they usually are.
Cheers!