Is it OK to just have two cutting boards: one for stuff to be heated, another for other stuff? Some say one should use more than one cutting boards because if you use just one board for raw meat and vegetables, bacteria from raw meat moves to the board and then to the vegetables, then you eat that vegetable raw, and you get sick.

Some uses more than two cutting boards: one for fish, another for raw poultry, yet another for dairy and so on. 

What if I use just two rather than 3 or more? One for stuff to be heated, another for everything else. What do I gain by using 3 or more rather than 2?
The exact number of cutting boards isn't critical, the important piece is minimizing cross contamination. You can use one cutting board safely as long as you're using it in a food-safe order (cut vegetables, then proteins), and follow good sanitation practices (wash &amp; sanitize the board between ingredients). A quick scrub with soap and water and a spritz with a bleach, alcohol, or quat solution between ingredients will minimize your need for different boards.