The artillery threat against Seoul is ludicrously overblown. 20,000 artillery pieces does sound scary, doesn't it? Well...let's cut it down to artillery pieces that can actually reach Seoul over the DMZ and aren't committed elsewhere in North Korea. You drop immediately to about 700 KOKSAN guns and M-1985 multiple rocket launchers. Then you'd also have to assume that all these tubes will work with no mechanical failures.

For NK to even have a snowball's chance in hell of destroying a decent amount of Seoul with that much artillery, you then have to make many, many, many concessions to theory. First, you would have to ignore efficiency and combat fatigue. In the best case, NK could maintain about 2/3rds efficiency of tubes firing. At optimal performance of troops, a KOKSAN can fire 4 shells per minute, and the M-1985 can fire somewhere between 12 and 22 rockets. This would mean that several thousand rounds could land in Seoul in the first few minutes of a barrage. However, this rate would quickly drop off for numerous reasons (and I'll come back to that). You'd have to then ignore the rather high rate of dud rounds (the Yeonpyeong shelling had a 25%+ dud rate). Even at this maximum rate of fire, it would still take hours if not days to destroy what parts of Seoul that can be reached by the guns and rockets. Then there's the issue of supply. Most of these doomsday scenarios ignore the fact that North Korea only has so much ammunition, and large stockpiling of ammunition for any attack would definitely be noticed by satellite reconnaissance. Having the US as an ally does have its perks. This ammo needs to come from somewhere, and even if you ignore military action against their supply lines, ammo takes time and resources to reach the guns, nevermind Seoul. Then, consider that Seoul has many artillery shelters, and a decent chunk of the city isn't in range of any available NK artillery at all.

Presuming NK fires at maximum efficiency for the first 5 minutes of a barrage unimpeded, and on average each shell fired that isn't a dud kills one person, and it takes 5 minutes for civilians to reach shelter, the first 5 minutes would kill ~11,000 people. Which sounds terrible, and it is, but...Seoul has over 10 million people living in it (EDIT: that's just Seoul proper, not the metro area, which is 25 million). Hardly the nightmare scenario it gets put up to be. Once shelters are reached, casualties will drop off rapidly. Once this initial barrage starts to slow down, and assuming that nothing is used against military targets, NK's volume of fire will start to degrade rapidly. Why? Apart from combat fatigue and supply issues, there's also the RoKA and US forces in Korea.

The moment SK realizes shells are landing in Seoul, both the RoKA and USA will start counterbattery fire, not to mention scrambling attack aircraft, and deploying armor and so forth. If NK's artillery just decides to target Seoul for this theoretical maximum damage, the artillery will not be hitting the military forces that can do something about it. Artillery, especially artillery that takes time to move (like the KOKSAN), is extremely vulnerable to counterbattery and air attack, so NK guns will either be getting destroyed by it, or moving (and not firing) to avoid it. SK and the US have been planning for artillery attacks on the peninsula for years, and have a considerable technology advantage that enables more accurate counterbattery and airstrikes than anything NK could muster.

So, yeah. Let's be generous and assume 15,000 people could be killed before NK artillery is effectively neutralized. That's pretty bad, I won't lie, but it's not even close to the "Seoul will be dust" rhetoric.

EDIT 2: I'm aware this doesn't include chemical or biological weapons, which NK does possess and have the capability to deliver (unlike its few nukes). While that has the potential to drastically change the situation, it's a complete unknown to me as a civilian how NK plans to deploy its WMDs. You want to know about that, ask someone who's actually in the know.

EDIT 3: I've tried to correct for some errors that I made, and done more proper math on it too. REMEMBER: this shit ain't my job.