One way you can achieve this is by using the command "last" which reads from the log file /var/log/wtmp. "last reboot" is supposed to show you the number of reboots since the log file /var/log/wtmp was created. The only problem with using this method is that some distros don't automatically create /var/log/wtmp, so if you did not create if yourself then you probably won't have any of the info that last is supposed to display.
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