Does only five survivors from a whole Century sound realistic?

[Jason Micallef]

I am writing a novel, and there is an opening scene in which a detatchment of an Auxillairy Century is accidently abandoned, and left to fight against impossible odds. I decided to have only around five men survive before help arrives. These five comrades, all from different conteburniums, are the main characters of the story. 
I just want to see if anyone thinks such a small number of well trained men surviving sounds realistic? I often debate with myself if this sounds plausible or simply too far fetched?

Any suggestions? Thoughts?