It looks like it is possible to have more than 5 dynasties if one republic conquers (by normal claim pressing CB) another republic: new vassal barons/mayors do not lose their patrician status.
https://forum.paradoxplaza.com/forum/index.php?threads/subject-republics.854188/#post-19270497
https://forum.paradoxplaza.com/forum/index.php?threads/subject-republics.854188/#post-19270497
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