If the wave theory of democracy is correct, then will the proportion of democratic countries increase or decline?

The proportion will increase and decrease at times.

Explanation:

There have been 3 recognized waves of democracy, a term that was popularized by the political scientist Samuel P. Huntington in 1987.

With each wave, dozens of countries have democratized their governments, but this number usually goes down a few years later and falls to a flat line.

For instance, in 1982 the first wave took place when 29 new democracies were formed after the fall of the Ottoman, German and Russian empires. However, only 24 years later, the number of democracies in the world was just 12.

As can be seen, the wave of democracy has its ups and downs, and it doesn't follow a reliable, constant pattern.