I've wondered about this for a very long time, but I don't know quite how to specify the question. I wonder how many people in the Roman Republic or Roman Empire at different times in their existences were Latins, how many spoke Latin, how many considered themselves Roman, how many were citizens, how many were descended from Italian or Roman or Latin colonists vs. assimilated natives vs. marriage between the colonists/conquerors and natives, what their physical features may have been like, and so on. Who became Latinized or "Romanized" and when? The Roman polity is something quite well-defined, but I don't quite see what a Roman person was.
Historical demographics are extremely tricky when you're looking outside the modern era, since detailed records on language, religion, population, and the like weren't kept as thoroughly as they are in modern nation-states.
I will point out that there was a massive part of the empire (the majority of the eastern region) that spoke Greek alongside the Latin West, which is why Biblical Roman subjects like the Apostles and Paul wrote in Greek. When the empire split, the Eastern part with its capital in Constantinople continued to be a Greek-speaking entity.
As for ethnicity, the areas of Western Europe that were conquered by the Romans (such as Gaul and Britannia) were primarily inhabited by people of Celtic stock, which later intermingled with Latin settlers. It's important to remember that the Germanic peoples didn't come onto the scene in large numbers until the empire was already in decline, so the people in Roman Britainnia were not 'English', nor were the people in Roman Gallia really 'French' (These groups are derived from the Angles and the Franks, respectively, which were Germanic peoples that later conquered these areas).
Source: My Roman History professor