Edit - I see Mongooz already weighed in on this, but;
It is a 9mm, but not the 9mm Luger / 9x19 / 9mm NATO cartridge that everybody knows.
This is a 9x18, which is somewhere between the 9x19 (9mm Luger) and the 9x17 (aka the .380) in muzzle energy.
It's about the most powerful cartridge that can be chambered in a blowback operated pistol without making the gun excessively heavy or impossible to pull back the slide. It's because the Makarov is a blowback design that it's so simple, rugged, and cheap.
One might expect ammo availability to be an issue, but there's a crap ton of it out there for cheap. Much of it's brass cased, and I reload.