When would the rank of Praefectus be used in the Late Roman Army

[Nathan Ross]

My understanding is that Numerus as a unit may have been a detachment of what the romans would consider ‘barbarians’ from allied tribes beyond the frontier ... (Before the 4th Century)

Yes, that's the earlier definition. By around AD300 numerus (or the Greek equivalent arithmos - both words just mean 'number') was used increasingly for newly-raised army units, both cavalry and infantry. By the late 4th century it was synonymous with 'military unit' and was used for everything from cavalry squadrons to legions.

roman units stationed in the North and West of Britannia were withdrawn around 383... If the ND dates from around 420 AD... some of the garrisons... could have mainlined their original roman names...?

A knotty question! Exactly who might have withdrawn troops from Britain, and when, and whether any came back again afterwards is pretty impossible to determine. Maximus may have taken some, Stilicho may have taken some more (but maybe introduced a new central field army under the Comes Britanniorum), Constantine III may have taken much of the remainder (or just the field army troops).

The ND list is probably based on the situation some time in the later 4th century, perhaps with additions from c.AD400. I would think it a safe bet that most of the units listed would have been present in the last quarter of the 4th century. It's also probably safe to guess that unit sizes would have been much reduced, especially in the aftermath of whatever chaos was going on c.AD367-69...

a list anywhere on the Internet of all roman army units listed in the ND and which specific regions they fell under and where they were garrisoned.

Yes! Luke Ueda-Sarson's excellent site had detailed listings for all the military commands in the ND (scroll down for the 'Western Empire', and further down for an alphabetical list of units).

seemed strange to me that a Late Roman Army legion on paper was almost exactly the same in size as the first Cohort of an earlier legion. This made me wonder...

You're not the first person to notice that!

However, the 'double strength' first cohort may not have survived the upheavals of the third century (or the departure of the primus pilus?). An tomb inscription from Sitifis in North Africa to signifer Aurelius Vitalis of Legion III Italica (AE 1967, 00639b) mentions that he's serving with a vexillation comprising cohorts I and II. The same place has a mithraeum built by men of cohorts X and VII of Legion II Herculia - these were probably legion vexillations of the mobile field force of Augustus Maximinian, operating in North Africa in AD298. So the first cohort was, it seems, the same size as the others by the end of the third century...

As these two-cohort detachments turn up quite often though, it seems likely that they might have been the ancestor of the later 'mini legions', rather than the old first cohort, or the auxiliary cohors miliaria.

My hypothesis would be that these detachments, originally comprising two cohorts of 480 men each, were merged into one large unit of 960 men and then subdivided into ten new 'mini cohorts' of 96 men, each commanded by an ordinarius. With officers included the new unit would be close to a thousand men. At some later date (under Constantine or his sons?) new senior soldiers named Augustales and Flaviales were introduced, bringing each 'mini cohort' up to c.120 men, and the complete legion to c.1200.