Artillery when was introduced and number of it

[D B Campbell]

He is asking when Artillery in roman legions were introduced (separately scorpio, ballista, onager) and in which number?

As you probably know, Josephus gives a snapshot of a three-legion Roman army engaged in siege operations during the Jewish War of AD 66-70. At Jotapata, Josephus claims that "Vespasian arranged the shooting machines all around, there being 160 devices in all, and ordered them to shoot at the men on the wall" (BJ 3.166).

Of course, Vespasian may have assembled a random number of catapults, but it's interesting to see how the total (160) tallies closely with Vegetius' information that "each centuria was accustomed to have a single carroballista", so that "In one legion there are usually 55 carroballistae" (ERM 2.25.2 & 4), and that "Ten onagers, which is one for each cohort, are carried around in ox carts, ready to use" (ERM 2.25.4) -- making 165 arrow-shooters and 30 stone-throwers in a three-legion army.

Vegetius probably drew upon an earlier source -- Book 2 is all about the "ancient legion" -- so he probably translated out-of-date terms (perhaps scorpiones and ballistae) into terms that his 4thC readers would understand.