It depends heavily on the region and the period, there are no universal convention like "if a ruler rules over 10000000 people he becomes an emperor".

In China, for example, certain words or characters are directly translated into a European title (although the same title is often translated differently depending on context), such as the title of huangdi being translated to "emperor" or the title of gong being translated to "duke." A huangdi is very different from a Roman emperor, and a gong has nothing to do with an English duke, but for non-experts' sake titles that have no true equivalencies in Europe are translated nevertheless. This can lead to confusion when people think a Chinese emperor is like a European emperor or that a Chinese duke is like a European duke - many Chinese "dukes" never ruled their "territory" in the same sense that William the Conqueror ruled Normandy, and it was just a title of prestige.

Likewise, a Powhatan paramount "chief" and a Taiwanese Aboriginal "chief" were very unlike each other, the only reason for the shared terms being that the English who encountered them developed a habit of calling leaders of non-state societies "chiefs." Then it gets confusing because a 1st-century Korean "king" probably had more affinities to our Powhatan than a European "king," but we call them "kings" because the Chinese had the different habit of calling various rulers 王 which in this context is translated by Western scholars as "king."

This trend of somewhat forced translation can apply more generally. You will often hear of Aztec "kings" or "emperors," when the actual term is "the great speaker" (huey tlatoani) and calling them "emperors" downplays the importance of the cihuacoatl, roughly speaking the head of internal affairs.

Sometimes nonexisting distinctions will be created. For example, we often refer to several Songhai rulers as emperors but several Dendi rulers as kings, although they had the same title of askiya. Sometimes existing distinctions are largely erased, such as the difference between various degrees of princes in historical Korea.

I'm not in my proper mind right now, hopefully this makes some sense.