1) How do you size HW requirements for ORACLE EBS for 200 Users, what would be the thump rule?
2) How do you size HW requirements for BI for 15 users.
3) Is there any know issue in running Oracle EBS +DB +BI on a VMWARE environment ?
4) What would be the best suitable OS ( Unix /Windows) for EBS and DB?
1- for PROD configurations, would suggest to go for proper hardware sizing exercise, you can refer following link for basic idea how to proceed.
3- Oracle's policy about VMWARE, refer
Hardware Vendor Virtualization Technologies on non x86/x86-64 Architectures and Oracle E-Business Suite [ID 794016.1]
4-would suggest you to do google, you will find number of articles on same topic as there are lot factors to take a call between however UNIX is excellent with compare to windows