Nikon Z 7 Reference Manual page 228

Ending Multiple Exposures
To end a multiple exposure before the
specified number of exposures have
been taken, select Off for Multiple
exposure mode or press the K button
followed by the i button and select
either Save and exit or Discard and
exit. If shooting ends or you select Save and exit before the
specified number of exposures have been taken, 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. Note that shooting will end automatically if the
standby timer expires.
D
The Standby Timer
Menus and playback can be used between exposures, but note that if
no operations are performed for about 40 seconds during playback or
90 seconds while menus are displayed, shooting will end and a
multiple exposure will be created from the exposures recorded to that
point. The time available to record the next exposure can be extended
by choosing longer times for Custom Setting c3 (Power off delay) >
Playback and Menus.
204 Menu Guide > C The Photo Shooting Menu