There was only one Mega Class Star Destroyer ever built, The Supremacy. This was Supreme Leader Snoke’s flagship. The colossal ship was over 60 kilometers wide and had a crew of over 2,250,000 personnel. It was so large that it could dock and carry 8 star destroyers.  It took 32 engines to move the ship, yet in spite of its size it could keep up with much “smaller” capital ships like star destroyers and Mon Calamari cruisers.

The Supremacy had 10 separate command facilities which could train, arm, and field entire armies and navies. Onboard were several manufacturing lines capable of mining asteroids and small planetoids, processing the ore, and building everything from hand weapons to AT-ATs and starfighters. 

It was eventually destroyed when Admiral Holdo rammed it with a star cruiser.  In an effort to protect the retreat of the Resistance fleet, she aimed her cruiser at the Supremacy and jumped to hyperspace. The collision destroyed 20 surrounding star destroyers and split the Supremacy in half. Due to its size and redundant systems, the flagship was still largely operational and continued pursuing the escaping resistance ships.