How Many Persian Immortals Were There?
Ten thousand, by Herodotus’ figure. He is explicit (VII.83) that the athánatoi were a closed-roster corps of exactly 10,000, perpetually maintained, and most subsequent Greek and Latin authors who mention the regiment — Ctesias, Aelian, Arrian, Quintus Curtius — repeat the figure with minor variations. Whether Herodotus is right, and what “right” would mean in this context, is a different question, and the modern scholarly answer is that the figure is approximately correct as an order of magnitude, but probably not exact, and probably not the regiment’s only organisational form.
The wider context is in the Persian Immortals cluster article and the Persian Military and Warfare pillar; the question of weapons and equipment is in the associated long-tail article.
What Herodotus actually said
“They had in their army a body of ten thousand men, all Persians, who were called the Immortals. They were so called because, if one of them was killed or fell sick, another was at once chosen in his place, so that their number was never more than ten thousand, nor ever less.”
That is the unit, in Herodotus’ report. Three things are worth noting. First, the unit is “all Persians” — Herodotus is explicit on this, and the Macedonian sources, writing a century and a half later, repeat the point. Second, the replacement is described as instantaneous, “at once,” which is a literary formula, not a logistical claim. Third, the figure of 10,000 is treated by Herodotus as exact: “never more than ten thousand, nor ever less.”
The replacement mechanism, the athanatos (literally, “not-dying”), is the source of the Greek name. The Persian or Median form behind the Greek title is, in the surviving corpus, not directly attested. The standard scholarly guess is something like anaušā, “immortal,” or a term alluding to the constant replacement, but the actual Old Persian word for the unit is not in the royal inscriptions, and we are working from a translation.
What the administrative evidence shows
The Persepolis Fortification Tablets, published by Richard Hallock in 1969 and continued by his students through the 2000s, give us a picture of the Achaemenid administration in the late sixth and early fifth centuries BCE that is detailed enough to support a unit of the size Herodotus claims. The tablets record the movement of grain, wine, and sheep to the royal household and to the workers and garrison personnel at Persepolis; the rations allocated to a guard unit of 10,000 men would be on a scale that the tablets can support. The tablets do not, in the published corpus, mention a 10,000-strong royal guard by that name, but the absence of a direct attestation is not a strong argument against the unit, given how fragmentary the surviving administrative record is.
The other piece of administrative evidence is the Achaemenid practice of organising forces in multiples of 1,000 — the hazārā, with a hazārpatish (a chiliarch) commanding each. A regiment of 10,000 would be a dah-yārā, ten chiliarchies, and the hazārpatish is well-attested in the Greek sources as the regimental commander of the Immortals. The Achaemenid state had both the demographic resources (a population in the tens of millions) and the administrative machinery to maintain a royal guard of that size, and the figure 10,000 is the right order of magnitude for the imperial bodyguard of a state of the Achaemenid scale.
The case for a smaller figure
The dissenting view, most strongly argued by Maria Brosius in The Persian Empire from Cyrus II to Artaxerxes I (2020), is that the actual strength of the regiment was smaller — on the order of a thousand — and that the athánatoi functioned as a royal bodyguard and strategic reserve rather than a full division. Brosius’s argument rests on three points: (1) the Greek taste for round numbers, of which 10,000 is a particularly tidy example; (2) the silence of the Babylonian, Egyptian, and Aramaic administrative documents on a 10,000-strong unit; and (3) the comparison with other Achaemenid units, which are typically attested at strengths of 1,000 to 5,000 in the surviving documentary record.
I think Brosius is right that the exact figure is doubtful, and right that the 10,000 is a literary tidy figure, but I think the order of magnitude is right. The Achaemenid state had the capacity to maintain a royal guard of 10,000, and the absence of a direct attestation in the surviving administrative documents is not, in a corpus of the size we have, a strong argument. My own best guess — and it is a guess — is that the regiment was maintained at a target strength of around 10,000, with the under-strength treated as a regimental matter, and that the “instantaneous replacement” formula is a literary construction rather than a logistical claim.
The number as a symbol
The figure of 10,000, in the Persian as in the Greek tradition, had a symbolic as well as a practical significance. The number 10,000 — in Greek, myrioi — was a standard figure in the ancient world for an unthinkably large or unthinkably perfect number. The Greek tradition, from the poets of the seventh century BCE onwards, used the figure of 10,000 as a way of indicating a great but undefined number, and the Persian tradition, with its own symbolic arithmetic, may have used the figure in a similar way. The Immortals, then, were not just a regiment of 10,000 men: they were a regiment whose number signified their status as the perfect, complete, and never-diminishing royal guard.
The symbolic significance of the figure is also reflected in the later tradition. The Ten Thousand, the body of Greek mercenaries who fought at Cunaxa in 401 BCE and whose retreat is described by Xenophon in the Anabasis, took their name from the Persian practice of organising forces of 10,000. The Greek tradition, more generally, used the figure of 10,000 as a way of indicating a body of men that was both complete and exclusive, and the use of the figure in the context of the Immortals was a way of marking the regiment as belonging to a special category.
The Achaemenid to Sasanian continuity
The Achaemenid practice of maintaining a fixed-size royal guard, with constant replacement, was continued, in different forms, by the Hellenistic successor states, the Parthians, and the Sasanians. The Seleucid kings maintained a somatophylax, or bodyguard, of fixed size; the Parthian kings maintained a guard drawn from the noble families of Iran; and the Sasanian kings maintained a guard of fixed size, later referred to in some Islamic-period sources as the pushtigbān (the Sasanian guard was, however, much smaller than 10,000 — a few thousand at most, and the surviving Pahlavi sources do not give a single figure). The continuity of the practice is a measure of the importance that the Iranian tradition attached to the royal guard as a symbol of legitimacy and as a strategic reserve. The Achaemenid Immortals were the model for a long tradition of Iranian kingship, and their institution has a continuity that extends well beyond the fall of the dynasty in 330 BCE.
What we don’t know
Two things would settle the question. One is a Middle Persian or Elamite administrative document listing the regiment by name and giving its strength; the Persepolis Fortification Tablets, in the published corpus, do not contain such a document, though new tablets are still being published. The other is a contemporary Achaemenid relief showing the regiment by name and size, with a casualty list; no such relief exists. Until either turns up, the question of “exactly how many” is not answerable in a strict sense. The order of magnitude — somewhere between 1,000 and 10,000 — is what the evidence will bear, and my own guess is the upper end of that range.