The administrative fee needs to be paid for each PC you will install office on. So if you are installing on 3 PCs you will need 3 licenses with three administrative fees.
Now if you replace one of the PCs, you can un-install office from that PC and reinstall the same license on the new PC, still limiting yourself to a total of 3 (or the quantity you order)
Also there is a 5 seat minimum requirement, so you will need to order 5 licenses of office, or 3 copies of office and 2 server CALs to meet the minimum order requirements.
NOW before you pull the trigger on the order, consider what your expansion needs are for the next year. Consider carefully as your opportunity for this donation is one time for the next 12 months,
I would suggest getting an extra copy of office for the shelf, in case you add a position with another computer in 9 months.
Also look at some of the other products that may benefit your operations like mappoint or project or visio this can get you some it would be nice to have software at a reasonable fee for the donation.
Remember you are receiving a donation of a $500.00 product for a $20.00 admin fee, providing a tremendous savings to your non profit org.
Dave