They are (probably) asking how to pad the page count of their final book before they send it to the printers, by adding blank pages at the end.
So this question is probably about asciidoctor-epub3 or asciidoctor-pdf.
Asciidoctor-pdf's publishing mode should get you part of the way there, but I don't know if there's a way to add blanks at the end:
https://github.com/asciidoctor/asciidoctor-pdf/blob/master/docs/theming-guide.adoc#publishing-mode(Printing presses print on extra-large pieces of paper called "signatures." These are folded and trimmed, so that one signature becomes 4, 8, or 16 pages of the final book. Thus the final page count needs to be a multiple of the signature size.)