What is the ruling on someone who made a covenant with Allah to fast three days if he were to miss a prayer for one day, and he has missed prayers for twenty days; does he then have to fast sixty days?

If you have made a covenant with Allah Almighty to fast three days for every prayer you miss, this is an oath of obstinacy (Nathr Lajaj). The majority of scholars hold that you have the choice between offering expiation for an oath (kaffarat yameen) or fasting the three days for every prayer missed. The expiation for an oath is feeding ten poor persons, or clothing them, or freeing a slave. If you are unable to do that, then it is fasting three days.