Nikon Z8 User Manual page 552

Ending Multiple Exposures
To end a multiple exposure before the specified number of
exposures have been taken, select [Off] for multiple exposure
mode. A multiple exposure will be created from the exposures
that have been recorded to that point (if [Average] is selected
for [Overlay mode], gain will be adjusted to reflect the number of
exposures actually recorded).
The multiple exposure will also end if:
the standby timer expires after the first exposure has been taken, or
you press the K button followed by the i button and select either [Save and exit] or [Discard and
exit].
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Multiple Exposure