In a hopping group (a set of frequencies to hop over) as defined by the MA (mobile allocation, i.e. which the frequencies are), there can be as many (actually BPCs) full-TCH channels as frequencies. The channels are denoted by the their “offset” (from zero) in that group. (offset = 0..number_of_freq-1)
The offset is actually a add (modulo) to make to the ARFCN (frequency compared with a “base-hopping frequency sequence”)