OIA only displays 500 records in the Identity Warehouse user list regardless of how many exist. One project i worked on had 70,000+ users in OIA. This isn't really a problem, because there seems to be little need to display all the users in the ID warehouse at once.
At least that's what I thought until I saw the same behavior on other OIA screens. For example, I needed to add 11,000 users to a role, but OIA only allows me to add a distinct set of 500. I can't choose the others after adding the first 500. OIA displays a distinct set of 500 users no matter what you do. Strangely, the set it presents includes a sample of users through out the alphabet. That is, it includes some users from A to Z. So I've got "aadams" through "zvincent". This is puzzling.
I found a setting in the config files that may affect this behavior in the idw-context.xml config file.
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I changed it to 5000 and restarted OIA. The result is that my 2880 user records showed up in the Identity Warehouse user search screens instead of the usual 500 users.