Always carry two - one for each hand
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Seriously though, usually just one though like to have a backup along when it's an option. Don't like leaving guns in the truck on day trips though, and when we camp we don't typically leave anyone in camp so same applies.
If hunting with a bigger group or an outfiltter I'd bring a second to stash in camp just in case. I keep my backup at the closest available secure location. More importantly, confirm zero before leaving and confirm torque on rings,bases, action screws etc.