Hi Faizan,
As mentioned by everybody, number ranges are maintained at the company code level, for a particular doc type.
Most of the time, Client demands certain things but they are not aware that the same thing can be achived in different ways in SAP. Why don't you show him a segment wise listing of the document numbers.. they may not be in serial No. but then all the documents posted for that Segment can be seen.
System should be designed as simple as possible. Until and unless it is a legal requirement, this should not be done.
Regards,
Kavita