Forty sheets of sky look identical face down on a table. Page numbers print a label in the margin of every tile so you can lay the poster out without guessing.
Set under Numbers in the export preview. Three options.
Off
No labels. Fine for a 2 x 2 poster, painful beyond that.
Column & Row
Each sheet is labelled with its grid position — the column number stacked over the row number, for
example C3 over R2 for the third column of the second row.
Use this one. It tells you exactly where a sheet belongs without reference to any other sheet, so a stack that got shuffled reassembles directly.
Sequence
Each sheet gets a single running number, counted left to right and then top to bottom. Sheet 1 is the top-left; on a 4-column poster, sheet 5 is the start of the second row.
Use it when you want the smallest possible mark, or when you are keeping the sheets in printed order anyway and just want to confirm none are missing.
Where the label is printed
In the left margin band of each tile, aligned with the scissor mark on the opposite side. It is inside the band you cut away, so it does not appear on the finished poster.
Like every other guide, labels are hidden when the margin is 0 — there is no band to print them in. See margins.
The preview matches the print
The label positions you see in the export preview are produced by the same code that draws them in the exported file, so what the preview shows is what lands on paper.