Late Roman Unit Sizes

[Renatus]

another interpretation I found from some years back in a discussion forum claimed it was five units.

its stated it was named after Augustus, yet the text also calls it the 'Unconquered', which would make it the Legio Palatina Undecimani... There are are three legionary cohorts...

The original latin is dictaque ab Augusto legio momenque probantes invicti. Loeb translated this (as part of a list) as "the legion, too, named after Augustus, that well called the Unconquered" - suggesting these are two units, not one. 'Unconquered' is surely Invictus (not undecimani, which I'm pretty sure Evan is right about

The problem is that the Loeb translation, no doubt for reasons of style, does not have enough 'ands'. Emphasising the 'ands' in the Latin text gives us:

Herculeam suus Alcides Ioviamque cohortem rex ducit superum . . . Nervius insequitur meritusque vocabula Felix dictaque ab Augusto legio nomenque probantes invicti clipeoque animosi teste Leones.

Adapting the Loeb translation to include these gives:

Alcides himself commands the Herulean and the king of the gods leads the Jovian cohort . . . The Nevian follows and the Felix well deserving its name and the legion named after Augustus and that well named Unconquered and the brave Lions as their shields witness.

On the 'Undecimani' issue, I am entirely with Evan and Nathan. There is no question but that it means 'the elevenths'.

Its information I came across in one of my many reference books

I don't know what reference book you are referring to, Adrian, but it is completely up the creek.