Hi Masood,
Your question will be very difficult to answer w/o having more details. In general it will depend on the workload. We can distinguish build (project / implementation) and run (support) activities.
Build activities have to be scoped (how many change requests) and include business requirements analysis, ETL, reports, dashboards, end-user training, etc.
Run activities can be estimated and are dependent on what is live and in support, how stable is the system, how many incidents occur, monitoring activities, etc.
Some other questions are: What does the company need? How big is the business user population?
My advice is to make a detailed analysis covering all aspects described above. I am sure that you are then able to make an educated estimate of the workload and the required SAP BW consultants (and do not forget the other disciplines) to handle the workload.
Best regards,
Sander