North Korea has 10,000 to 12,000 artillery pieces and 2,300 MLRS pieces over 107mm. The majority of tube artillery are 122-, 130-, 152- and 170-millimeter units, and on the rocket side the majority are 240-millimeter units. (per google search)
Some experts estimate they could fire 500,000 rounds in the first hour. The tube artillery would need rocket assist to hit Seoul proper but the outskirts are certainly within range. Those same experts predict a high dud rate for the artillery so perhaps a lower amount would actually do damage. On top of that they have significant stores of chemical and bio weapons which can be delivered with the artillery.
The THAAD is designed to be used against missiles and not the above artillery. SK and the US have significant forces in place to take out the artillery but that would take time.