What is a printing number line?

The printing number line is that row you've seen on copyright pages — 10 9 8 7 6 5 4 3 2 1. In traditional publishing, the lowest number showing indicated which print run a copy came from (the printer removed a digit each run).

For print-on-demand (KDP, IngramSpark), where every copy is printed individually, it doesn't really mean anything — there aren't distinct "print runs." So it's optional and usually unnecessary; plenty of self-published books skip it. Include it only if you like the traditional look. The generator makes it optional.