From The Truth About Guns:

One of the most hotly debated issues on the Second Amendment is over the right to carry firearms outside the home.  Today, each of the 50 states as well as the District of Columbia authorizes ordinary citizens to carry a handgun in most public places under one of the following three regimes:

• Constitutional Carry: 12 states
• Shall-Issue permit: 30 states and DC
• May-Issue permit: 8 states