I pretty much suck at miniature football this year, but I'd give this topic my two cents worth.

Linemen: 8 (5 just for offense)
Backs: 7 (3 quarterbacks to give you run-pass options)
Receivers: 5 (all set up to run different routes)
Cornerbacks: 5 (all lockdown-type guys)
Linebackers/Safeties: 5 (speedy guys on ttc bases for zone coverages)
Special teams: 3 (kickoff specialists like return speedsters & bullets to cover)