in the elements: The maximum number of target users to be listed in the User box in the Act As dialog box. If the number of target users for a proxy user exceeds this value, an edit box, where the proxy user can type the ID of a target user, is rendered rather than a drop-down list of target users. The default is 200.For example between the node, you can insert: 
<LogonParam>
  <TemplateMessageName>LogonParamSQLTemplate</TemplateMessageName>
  <MaxValues>100</MaxValues>
</LogonParam>
The name that you specify in the element must match the name that you specify in the element in the custom message file.
Assigning the privilege and restarting the BI Presentation Service
For each user whom you want to authorize as a proxy user or for each Presentation Services group whose members you want to authorize as proxy users, you need to assign the Proxy privilege.And then to load the custom message and the configuration file changes, you must restart the BI Presentation Service .
How to
See the proxy level
You can check the proxy level in the “My Account” > Delegate Users tab.Obiee 11g My Account Delegate Users
Support
Unable to sign-in
Check the nqserver.log. You may find this kind of error: 
[2012-09-24T16:43:27.000+00:00] [OracleBIServerComponent] [ERROR:1] [] [] 
[ecid: ae920880d33cb0ac:-1efbec60:139e4853ab6:-8000-000000000000dea0] 
[tid: 4587e940]  
[nQSError: 13022] There is no init block for PROXY session variable.
See creating_session_variables_for_proxy_functionality
Documentation / Reference