Before increasing the number of BI Servers per server, make sure you have enough physical memory available on 32bit / 64bit servers and during performance / load testing, adjust as needed. For a stable / an even performance over time, it is recommended to set minimum two BI Servers per instance / server.
You can use the below Scalability tab of the Capacity Management page in Fusion Middleware Control (http://<servername>:7001/em) to scale in the Oracle BI Server to two per instance and server:
Important Note: When you have multiple instances of a given Oracle Business Intelligence component in the deployment, you should first configure shared files and directories for the clustered components to use. Then, use Fusion Middleware Control to increase the number of BI Server components that run on the existing Oracle Business Intelligence host.