One PC may have up to two HMI servers. Functionally I don't believe it will prevent you from adding a third. I seem to recall running more than two HMI servers for demonstration purposes.
But, two is what Rockwell supports. If you are running more, and have an issue, they may say you are not running a supported configuration.
Having support for two was primarily to support redundancy. HMI A could be a primary server for say the bottling process while also being a redundant backup for the quality control process. HMI B then would be a redundant backup for bottling while being the primary for quality control. But they could also be two separate, non-redundant, HMI servers.
OG