However, digs at several long-term legionary forts in Germany and the UK have found the remnants of facilities for mass producing pila and ballista darts, and it would not be terribly surprising if they could also crank out large numbers of arrows for the auxilliaries.
And the Iranian, who claimed he could darken the sky with arrows?
The logistics would be complicated and difficult to sustain.
The Romans weren’t big on cavalry either. I believe many of their horsemen were auxiliaries also. Heavy infantry was their forte.
I recall reading Greek and Roman slingers used to carve insults on their lead sling bullets.
Freegards
[Strangely, the Romans were not much into archery or bows - missile troops tended to be drawn from auxilliaries from conquered provinces, client states or even mercenary formations.]
Slingers were used by the Romans with great effect. The Balearic Island slingers, trained with the sling since childhood, slung stones weighing a pound or more. The stones didn’t pierce armour but the impact imparted enough kinetic energy to sometimes cause fatal injuries.
FROM WIKIPEDIA:
(Balearic Islanders). . .were famous for their skill with the sling. As slingers, they served as mercenaries, first under the Carthaginians, and afterwards under the Romans. They went into battle ungirt, with only a small buckler, and a javelin burnt at the end, and in some cases tipped with a small iron point; but their effective weapons were their slings, of which each man carried three, wound round his head (Strabo p. 168; Eustath.), or, as seen in other sources, one round the head, one round the body, and one in the hand. (Diodorus) The three slings were of different lengths, for stones of different sizes; the largest they hurled with as much force as if it were flung from a catapult; and they seldom missed their mark. To this exercise they were trained from infancy, in order to earn their livelihood as mercenary soldiers. It is said that the mothers allowed their children to eat bread only when they had struck it off a post with the sling.[12]