If you only need to print half a sheet (and not one and half sheets, or to
put it in order, half a sheet and then a whole sheet), in the mailmerge main
document, you can delete the mailmerge information that appears in the
"labels" that have been used.

If you need to print more that the remaining half sheet, then you can do it
by adding the requisite number of empty records in front of the first record
in the data source.