The Mushroom Kingdom is a monarchal governed state that which is almost always the main setting where SMG4's bloopers take place. There are many different species that reside in the kingdom including Toads, humans, Goombas, Koopas, and so much more.
Its population is around 15 million people, since at the beginning of SMG4: Mario and The Diss Track, the counter of Bob's Fans is around that number, while Saiko said "everyone in the Mushroom Kingdom are already fans of him".
In the Nintendo counterpart, Bowser rules the Koopa Kingdom, but in the SMG4verse, he and all Koopas seems to live along with the people in the Mushroom Kingdom.
The kingdom has been through numerous apocalyptic events; too many to even count.