If your data is in Geographic Coordinate System(WGS84 etc..), the units will be in decimal degrees(angular). So I would project the polygon shapefile first to a Coordinate System with Meter as unit(like NAD83 UTM Zones..). Then the buffer distance will be in meters.

With a Quick google Search, find the conversion between Nautical mile and Meter.

    12 Nm = 22224 meters

Now Run `Vector > Geoprocessing > Buffer`
give 22224 as buffer distance

An excellent answer is here https://gis.stackexchange.com/a/23863/5850


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