Actually, an operator in a country can use any BSIC/NCC and should do so except where there is possibility of overlap with co-channel border PLMNs.
You will have to adjust the ‘NCC permitted’ or ‘PLMN permitted’ (whatever its called in your BSC)parameter accordingly at those border cells.
You can use the all NCCs including those of any other non-cochannel operator in the same region too without any problem.
If you use the same single NCC everywhere in your network then you are likely to get co-BCCH co-BSIC neighbour of neighbour problems that appears to cause lots of HO failures.
The NCC part of the BISC is one of the most misunderstood parameters. It is only used to prevent mobiles in co-channel PLMN border regions reporting BCCHs that don’t belong to the home network.