i want answers to the following questions wrt both user based and core based server.
1. How many concurrent user can tableau server handle with 4 to 8 cores and 16 GB ram with 64 bit system.
2. For 200 concurrent users, what tableau server requirements are needed. actual users will be more.
I'd contact Tableau Sales, they can give you the best answer.
Take a read of this - Planning Your Tableau Server Deployment
There are more factors to consider including frequency of refresh, number of workbooks etc. All of this would be surfaced with a conversation with Tableau Sales.