What is the population of Northeastern Mexico?

It depends on what "Southern Mexico" means to you. The most commonly associated states to such region are the states of Guerrero, Oaxaca and Chiapas:

• Guerrero: 3,388,768
• Oaxaca: 3,801,962
• Chiapas: 4,796,580

Total population for southern Mexico (according to the 2010 census) equals 11,987,310 people.