What does it mean on the box when it says

Fantasy Flight Supply

x

Protective Card Sleeves

and x is a number?

Regards

WombatMan

It is the number of packets of the FFG Mini American size sleeves that you will need to sleeve all of the small cards in a particular expansion.

The City (above) requires 4 packets of these (each has 50 sleeves)

Details of the colouring of the sleeves and sizes can be found here -

http://www.fantasyflightgames.com/edge_minisite_sec.asp?eidm=86&esem=4