EDAP is Abis resource(of Nx64Kbps timeslots) for transmission of packed data. It is being used not only for EDGE, but for CS3/CS4 GPRS traffic also.
EDAP should be attached attached per BCF, so it means, that one EDAP is necessary for one BCF, if you have at least 1 TRX with CS3/CS4 or EDGE.
Size of EDAP depends of packet traffic of all BTSs under BCF.