of course you need an Assistant Manager and a good U/21 Manager as well...
the others (to my knowledge) really add nothing at all... maybe only for larger/longer tournaments when they need to keep fit (physio's) and do not go back to their club for a couple of weeks etc...
other than that, I dont believe they add much, even though I try and hire the best of the best (99% of the time, they are already employed by me at my club)