We never reach such amont of user, but the reason come from the size of the community that we managed that were around a maximum of a thousand of users.
But there is no reason that Libresource will break above. Mainly, it will depend how your user will stress the server and if they always work together at the same time. The Libresource synchronizer for exemple put the load on the client side, not on the server so this should scale. For the rest of the resource it will realy depends on your community and the way you organized your server.
Anyway, I hope that some other user that are using libresource will post there experience with there own install...
Seb