The Shibboleth EDS by default sets maxPreferredIdPs = 3 and includes the comments that "The following should not be changed without changes to the css. Consider them as mandatory defaults".

This effectively limits the number of preferred IdPs that can be down in the Registry embedded EDS to 3.

The Registry could instead, based on the number of preferred IdPs that the CMP admin configures, dynamically adjust the CSS and maxPreferredIdPs to accommodate more than 3.