I was reading the Wikipedia page on Roman naming conventions last night, and it was mentioned that various aspects of naming conventions at different points during the Roman period mean that several individuals share names, and this can make it hard to tell which individual is being referred to in a specific source. This set me wondering about the number of individual Romans whose existence is known about through whatever source to modern historians.
I understand that it's probably not possible to give an exact figure, but I'm curious as to orders of magnitude - do we know of hundreds of individual Romans? Thousands? Tens of thousands? And roughly what proportion of all the Romans who ever lived do we know of as individuals - i.e. what proportion of individuals from Roman civilization are we basing our knowledge of the culture of the entire civilization on, and what percentage of individuals are lost to us?
I realise that there are other non-personal sources of information about aspects of Roman history, culture and life, but I'm particularly interested in knowledge we've gleaned directly from sources pertaining to specific individuals, and how many of those individuals we know of.
It depends what range of dates you would like to cover, but the order of magnitude is generally in the tens of thousands.
The Prosopographia Imperii Romani is a German reference work in three volumes that attempts to list every known Roman person who is attested by name. These are generally senators and equestrians, since they tend to dominate the surviving sources, but there are also some other ranks, such as freedmen. It is organised alphabetically, with each letter listing the corresponding names with an associated number - so, for instance, IMP. CAESAR M. AVRELIVS COMMODVS ANTONINVS AVGVSTVS (i.e. the emperor Commodus) is A 1232. The letter A alone has 1359 entries, and while some letters certainly have fewer entries than that (e.g. B has 157), the total number of entries adds up fairly quickly.
The original volumes are quite old now, having been published in 1897-1898, but there has been an ongoing effort to update the PIR since 1933. I believe they are working on the letter T at present, so they haven't quite finished yet - but they're nearly there! Thus, giving a completely accurate total isn't quite possible yet.
Similarly, the Prosopography of the Later Roman Empire provides a comparable resource for the years 260-641.
Our main knowledge about individual Romans (and non-Romans!) comes from inscriptions. We know about 460.000 individual inscriptions, many of which contain multiple names, sometimes tens and more, many of which contain no name at all - and not all of them are Roman.
To my knowledge, there has been no comprehensive and complete study done on this. The most complete database (the Clauss-Slaby epigraphic database) contains no additional information as to the individual persons, only the naked inscription with a few references, while databases such as the EDH-database, which aims to comprehensively analyze and file the inscriptions with entries for individual persons or names only cover a small area as of yet (EDH so far contains ~67.000 inscriptions, many of which are only preliminarily edited).
The PIR /u/Justinianus mentioned is a bit dated, and mainly a prosopographical work, not an onomastical (meaning it's concerned with individual persons lifes more than names). In the 1980s there have been efforts, most prominently by András Mócsy at the University of Budapest to create a Nomenclator Provinciarum Europae Latinarum et Galliae Cisalpinae, with the support of modern computers, sorted by Province (with assorted quadrant) and type of Name. . This work was tragically cut short by his death, but at the end his database contained about 44.000-45.000 names (Milena Minkova quotes "more than 80.000 persons" [Minkova 2000, p.8]) by Mócsys own estimate.
Building on that work, we got the OPEL, the Onomasticon provinciarum Europae latinarum (Budapest 1994-2005), the onomastic database for the Latin Provinces of the Roman Empire, which contains more names (I couldn't find out how many online, library's closed and I'm not going to count them all...).
However, this work has been massively criticized both for methodological flaws (such as listing different abbreviations of the same names separately; not including particular corpora of names) and factual errors (such as in the emendation of abbreviated names and the geographical placement, and sometimes missing published names). It also doesn't tell us much about the concrete individual a name belonged to. It is a naked list of known names, not of Persons.
A complete and comprehensive lexicon of Roman personal names is still a desideratum.
We 'know' probably multiple hundreds of thousands, however this knowledge is as of yet not sufficiently analyzed and edited to be available in an easy form. If you want, you can have a look at what's available
here:
http://db.edcs.eu/epigr/epi_en.php
and here:
http://edh-www.adw.uni-heidelberg.de/home?&lang=en
There have been multiple hundreds of millions of people who ever lived in the Roman Empire and Republic during its history, so we only know fractions of a percent.