What you’‘re asking for by multiple Jabber servers supporting the same domain is essentially clustering, and the answer is
currentlyno, openfire doesn’'t support clustering–yet.
However, Openfire does support connection managers. Connection managers are essentially a multiplexer of OpenFire IM. They combine requests and pass them on to the server as a big chunk rather than many little chunks. The result is less connections to the actual IM server and thus less memory and threads utilized on the IM Server. The Connection manager application itself isn’'t doing a lot of processing work so the utilization is rather low as well.
I thought I remember somewhere seeing 1 connection manager per 10k users, but I can’‘t seem to find the reference anywhere. From an availability and scalability perspective, you could put 2 connection managers up behind a load balancer and have your clients connect to the connection manager in a round robin or least used fashion and openfire should scale nicely. As needed, add more CMs and you’'re golden.