Dirt is found at any altitude, and comprises the majority of the upper terrain layers in most overworld biomes.
There are approximately 1,850 dirt blocks per chunk in Plains, Forest, Tundra, Jungle, and Extreme Hills biomes. There can be as many as 3,000 in chunks with high mountains. Dirt generates in pockets underground as well. Dirt also generates naturally in the butcher's shop and in some houses in villages.
Coarse dirt can be found in mega taiga, mesa and savanna biomes.