§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.
§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.
Source: Code of Canon Law (Vatican.va)