- If the hex is a safe haven or civilized outpost or otherwise relatively safe place, roll 3d4 for encounters;
- if it's borderlands, roll 2d6;
- if it's dangerous wilderness, roll 1d12.
Then you just use your knowledge of where the peaks and impossible results are on the 3d4 and 2d6 rolls to your advantage when designing the table.
The 1-12 range is there for two reasons. One, it fits on the page without cramming things in; and two, it allows you to have some kind of coherent theme for the region. You have twelve entries to work with, so there will be repetition if the players spend much time in the region. This repetition is desirable. It sets a consistent backdrop for what the region is like.