Questions on the deployment of auxiliaries and the praefectus castrorum's role

[Steven James]

Nathan wrote:

There's a common conception that Roman campaign armies tended to be split approx 50/50 between legionaries and auxiliaries - I'm not sure where the idea comes from…...

 

Apparently it comes from Tacitus. Sorry I don’t have the reference, and the last time I examined this reference, I did not find it open to doubt. However, from this opaque reference many academics have based the concept of the auxiliary and the legionaries as being a 50/50 split.