I normally run 12-16 models in my Legionnaire units. Don't forget that Auxiliaries have the Legion Rule
also, so can fill in a second line for support. I tend to like the way it works, and my normal mix of units is approximately 50-50 with Legionnaires and Auxiliaries. The Legionnaires are tough as nails, but if you spend too many points on them, your army ends up being very small indeed.