GMAT Critical Reasoning (CR) | Monarchy, a stable form of government for many centuries,

the difference lies between many and most i.e. degree of certainity. If many countries had regime changes, doesn't mean that most had the changes.
As for E, suppose in the 18th 100 countries were monarch, in the 20th, many had regime changes, suppose 120. so it is more likely that a person lived in a monarch country, if he was living in 18th century compared to the 20th.