Possible Clues:

• Pope during World War II
• Name shared by a dozen popes
• 12-time Vatican name
• Apt name of 12 popes
• __ XIII: Title role in 'The Young Pope'
• __ XIII: Title role in "The Young Pope"
• Rome’s increasingly spoilt, at first, for choice of popes?
• Dutiful, to Diocletian
• Name of twelve 34 Down
• Twelve-time papal name