"Janus the fifth" and "KD6-3.7"
"The fifth slave I bought" and "the fifth son of this super rich senator" would have exactly the same name, Quintus.
Pompey Magnus had a son, Sextus (6th), that was relatively famous for his opposition to Octavius.
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.