The uniqueness tuple for IP is (source IP, source port, dest IP, dest port). You're limited to 65,535 connections from the same two IPs on the same port, but that's not relevant to XMPP which uses only one and some transient things for file transfer and such. At worst having that many people behind one NAT will be a problem... which at this scale could be an actual problem, but there are still solutions (multiple ports being the easiest, and the fact this cluster will probably be on multiple public IPs anyhow).

See https://ats1999.github.io/blog/65535/ or similar pages.