After arcgis server is installed, the pooled "maximum number of instances" defaults to 2. The maximum number of instances can be set in the Pooling tab of the "Service Properties" dialog box. The principle of adjustment is: First, the number of instances should be set equal to N+1, where N is the number of CPU cores on the computer. In individual cases, if you find that the CPU utilization on your machine is not approaching 90%, you might consider adding more instances.
http://help.arcgis.com/zh-cn/arcgisdesktop/10.0/help/index.html#/na/00530000009r000000/
For other deployment and parameter requirements, please refer to:
http://help.arcgis.com/zh-cn/arcgisdesktop/10.0/help/index.html#/na/000400000006000000/