This is likely outdated advice.
It depends on what you're doing with the cases and your preference. If 18 cases is enough, and you think they will last the match out, then 18 cases is good. If you are losing 1 or 2 every target because of something like split necks, you need to be fire forming new cases right then and there. You need a couple of smaller neck sizing bushings to take care of work hardened necks. These suggestions will get you to the end of a match. What you do when the match is over is up to you.