Canon 905

Can. 905 §1. A priest is not permitted to celebrate the Eucharist more than once a day except in cases where the law permits him to celebrate or concelebrate more than once on the same day.

Can. 905 §2. If there is a shortage of priests, the local ordinary can allow priests to celebrate twice a day for a just cause, or if pastoral necessity requires it, even three times on Sundays and holy days of obligation.