How many days should tarpan be performed during Pitrupaksha?

Ideally, tarpan is performed on all fifteen days of Pitrupaksha. The most critical days are the opening Purnima Shradh, the tithi matching the ancestor death anniversary, and the closing Sarva Pitru Amavasya. If you can only observe one day, Sarva Pitru Amavasya is the most inclusive.