NK has over 5000 artillery pieces 122mm and greater. Rate of fire depends on the system, but figure most of them can fire at least 5 rounds a minute for a few minutes before slacking off and/or suffering from counter-battery fire. So that is a possible 25,000+ rounds per minute for, say, 10 minutes. Thats 250,000 rounds. Granted, not nearly that many guns would actually be in place and firing, but it does show the numbers nicely.
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Artillery is know as The King of the Battlefield for a reason.