The Companion Cavalry was a famous group of horsemen in Ancient Macedonia. They served quite literally as the companions of the Macedonian kings during battle.
Alexander the Great himself rode with them many times during the wars in the east against the Persian Empire from 334 B.C. till his death some ten years later.
According to Nicholas Sekunda the Companion Cavalry was divided into eight squadrons:
The squadron or ile, commanded by an ilarches (Arr. 1.12.7), numbered 200 Companions (Arr. 1.18.1; cf. 4.17.3, 22.1).19 When the army crossed into Asia the Macedonian cavalry numbered 1,800 (Diod. 17.17.4), and there were eight ilai, so we must presume that the ‘Royal Squadron’ (basilike ile) was double-strength
From: Roisman, Joseph, and Ian Worthington, eds. A Companion to Ancient Macedonia. Vol. 84. John Wiley & Sons, 2011
In Arr. An. 3.11.8, however, there seem to be multiple royal squadrons present at the battle of Gaugamela. (emphasis mine)
τὸ μὲν δεξιὸν αὐτῷ εἶχον τῶν ἱππέων οἱ ἑταῖροι, ὧν προετέτακτο ἡ ἴλη ἡ βασιλική, ἧς Κλεῖτος ὁ Δρωπίδου ἰλάρχης ἦν, ἐπὶ δὲ ταύτῃ ἡ Γλαυκίου ἴλη, ἐχομένη δ᾽ αὐτῆς ἡ Ἀρίστωνος, ἐπὶ δὲ ἡ Σωπόλιδος τοῦ Ἑρμοδώρου, ἐπὶ δὲ ἡ Ἡρακλείδου τοῦ Ἀντιόχου, ἐπὶ ταύτῃ δὲ ἡ Δημητρίου τοῦ Ἀλθαιμένους, ταύτης δὲ ἐχομένη ἡ Μελεάγρου, τελευταία δὲ τῶν βασιλικῶν ἰλῶν ἧς Ἡγέλοχος ὁ Ἱπποστράτου ἰλάρχης ἦν. ξυμπάσης δὲ τῆς ἵππου τῶν ἑταίρων Φιλώτας ἦρχεν ὁ Παρμενίωνος.
His right wing was held by the Companion cavalry, the royal squadron in the front; it was commanded by Cleitus son of Dropides; in successive order came those of Glaucias, Aristo, Sopolis son of Hermodorus, Heraclides son of Antiochus, Demetrius son of Althaemenes, Meleager, and lastly that [of the royal squadrons] commanded by Hegelochus son of Hippostratus. The Companion cavalry as a whole was commanded by Philotas son of Parmenio.
Translation by Rzepka, Jacek. "The units of Alexander’s army and the district divisions of late Argead Macedonia." Greek, Roman, and Byzantine Studies 48.1 (2008): 39-56.
The text between brackets was added by me to make clear where Arrian's text uses the term "royal squadron" again.
Note: The use of plural here seems unusual, especially when reading Arr. An. 3.8.1.
Here Arrian clearly speaks about the royal squadron as a singular unit. Next to that he also refers to one regular squadron of the Companion cavalry.
ἀναλαβὼν οὖν τήν τε βασιλικὴν ἴλην καὶ τῶν ἑταίρων μίαν καὶ τῶν προδρόμων τοὺς Παίονας ἤλαυνε σπουδῇ
Alexander therefore took the royal squadron of cavalry, and one squadron of the Companions, together with the Paeonian scouts, and marched with all speed.
Translation from The Anabasis of Alexander; or, The history of the wars and conquests of Alexander the Great. (1884) by Arrian, translated by E. J. Chinnock
The question: Why did Arrian refer to the royal squadron of the Companion Cavalry as multiple units in this specific case?