I have 20,000 external Users, they don't have domain account. How can i create User Profiles for them in SharePoint 2010 ? Is there any way to create them without adding them to domain ?
You can manually create user profiles from the "Manage User Profiles" screen in Central Administration (though for 20,000 you'd likely want to automate it with PowerShell.
All profiles have to be associated with "an account", but that account doesn't have to be an AD account. How are your external users authenticating? As an example, if you're using claims/FBA, with a SQL database containing the user accounts for your external users, you should be able to create a user profile using the claim provider and account.
In order for it to work correctly, you'll have to make sure you've properly completed the web.config changes for STS and CA on the Central Administration server, not just on the WFE(s).