What should a wife do if she forgets the number of fasting days she owes?

Days missed in Ramadan due to an excuse must be made up, as Allah Almighty says: {But whoever among you is ill or on a journey [during them] - then [he must fast] an equal number of other days.} (Al-Baqarah: 184). And whoever forgets the number of days to make up should take the lesser certain amount, as the fundamental principle is the acquittal of one's liability. However, it is more cautious to fast the days about which one is doubtful. If it was an obligation, her liability is certainly absolved, and if it was not an obligation, then it is a voluntary act.