If you're using campaign ops rating, page 21 in campaign ops has the rules for admin personnel. The ratio of admin personnel to others affects your campaign ops unit rating - if you don't have enough, then the rating takes a hit. If you don't want to hire fifty bazillion admin people, you may consider giving people a secondary admin role, until that rating penalty goes away.
For practical purposes, if you're playing AtB, you want at least one doctor, and at least one of each admin. If you're acquiring a lot of parts, you may consider either hiring additional admin guys or, as mentioned before, giving some of your techs/mechwarriors secondary admin roles.