> Can anyone explain this?

Slave names were sometimes numbers. Naming conventions in Roman culture were apparently also fairly formal.[0] I would guess there's sufficient evidence here that a ratio of free to slave could be estimated.

In general, it seems like being a slave was not a great experience.[1]

[0]: https://en.wikipedia.org/wiki/Slavery_in_ancient_Rome#Treatm...

[1]: among other stories, the one concerning the murder of Lucius Pedanius Secundus sticks in my mind: https://en.wikipedia.org/wiki/Lucius_Pedanius_Secundus

> it seems like being a slave was not a great experience

It was definitely frowned upon.

"Janus the fifth" and "KD6-3.7"

That's not at all how it worked.

"The fifth slave I bought" and "the fifth son of this super rich senator" would have exactly the same name, Quintus.

Cicero’s less famous brother was named Quintus.

Pompey Magnus had a son, Sextus (6th), that was relatively famous for his opposition to Octavius.

Not sure precisely why you're being downvoted for bringing some additional light to the conversation. Been busy here but appreciate the insights; this isn't my field. Are there good sources that clarify the complexities of Roman naming conventions?

> "Janus the fifth"

You're thinking along the lines of generational suffixes? There were Roman emperors who's family name (nomen gentilicium) was a number, the same number being used by son, father, grandfather... not incrementing with each generation.

There were also Roman emperors who's fathers had been slaves.

Lucius Septimius Severus - family name means 7th

Publius Helvius Pertinax - family name means honey-yellow. Son of a freed slave.

(Even?) today there are many people involved in the sex industry voluntarily - I know of a Doctor who has left her specialist job to pursue full time sex work, she says it pays more. It’s not so hard to believe that some women 2,000 years ago would have made a good living in a society where this kind of work was accepted and legal.

Presumably a portion could have been previously slaves who earned their freedom; and maybe even some who chose it as a line of work as free women initially

Just because that kind of "work" is legal doesn't mean that it's accepted.

Thanks but what I'm asking is how they can tell from their names? Is there a naming convention here?

There were naming conventions and fashions that changed over the centuries. Most importantly, slaves had only one name, while Roman citizens or freedmen had two or three.

I need a clarification:

You know a medical doctor who left medicine and is working as a prostitute because it pays more?

What country if you don't mind me asking?

Restituta may suggest a freed person (and would have probably not been given to a slave), and "the daughter of Salvius" provides a linage suggesting a free citizen. But these are only two, no idea what the others may be.