A mental model is just a concept you can use to help try to explain things (e.g. Hanlon’s Razor?—?“never attribute to malice that which is adequately explained by carelessness.”). There are tens of thousands of mental models, and every discipline has their own set that you can learn through coursework, mentorship, or first-hand experience.
There is a much smaller set of concepts, however, that come up repeatedly in day-to-day decision making, problem solving, and truth seeking. As Munger says, “80 or 90 important models will carry about 90% of the freight in making you a worldly?wise person.”