Q: What should be done if the required number of attendees cannot be met for Jumu'ah?
A: If the Imam believes that a specific number of attendees is a required condition for Jumu'ah prayer which cannot be met, so long as the congregation does not hold the same view, it is not permissible for him to lead them in prayer because he will be performing something he believes to be invalid.
Rather, he is required to appoint someone who does not believe that number is required so that the people can fulfill their obligation.
If the congregation holds that view while the Imam does not, neither they nor he are obliged to pray the Jumu'ah prayer. The Imam is not required to do so because he will have no one to pray with and the congregation because they believe their Jumu'ah is invalid. Instead, they pray Zuhr.
Source: Kashshaf al-Qina, Sharh al-Muntaha, al-Insaf