Sam,
The BCCH is located allways on TSL0 on the TRX carried BCCH frequency. The SDCCHs allocation is depending on the vendor’s requirements, but generally it is recommended to be placed on the BCCH ARFCN but this is not mandatory. The number of the SDCCHs depends on that what is the number of TCHs in the cell, full-rate or half-rate, what is the type and location of the cell in the network and many more. There are tables from which you can dfine the initial number of SDCCHs and after performance measurements you can tune it.