number of ARFCNs in a BTS depend on many things, such as:
number of cells of your BTS,
number of TRXs of each cell,
and frequency hopping(if used).
each TRX (BCCH or TCH) must have 1 ARFCN
if FH is used (e.g RF FH), each non-BCCH TRX must have two or more ARFCNs,