There is a post floating around about a production instance of OpenERP that has 150,000 users, 100 http queries per second, and over 1,000,000 product codes. I could not verify the origin.
I can tell you that 1000 users, on a non-media rich implementation would not be hard at all. Run the web app on something like ubuntu 12.4, and run the postgresql on a DB server instance separate from the application. If you see repetitive queries slowing down response time install an instance of memcached on a server for quicker reference times for users.
You could probably even do 1000 users on a stack depending on how many concurrent users and queries you would see.