What is required of the one who did not fast some days in Ramadan because of an excuse such as travelling, sickness, menstruation or postpartum bleeding (nifaas) is to make them up, because Allah says (interpretation of the meaning):
“but if any of you is ill or on a journey, the same number (should be made up) from other days”
Muslim (335) narrated that ‘Aa’ishah (may Allah be pleased with her) was asked: Why does a menstruating woman have to make up missed fasts but not missed prayers? She said: That used to happen to us and we were commanded to make up the fasts but we were not commanded to make up the prayers.
If your wife has forgotten the number of days that she owes, and is not sure whether it was six or seven for example, she only has to do six, because in principle she does not owe any days (except those she is certain about). But if she fasts seven in order to be on the safe side, that is better, so that she can be certain that she has fulfilled the obligation.
If she does not remember anything about the number of days she owes, then she should fast whatever she thinks most likely to fulfil the obligation.
Shaykh Ibn ‘Uthaymeen (may Allah have mercy on him) was asked: There is a woman who has to make up days from Ramadan, but she is not sure whether it is four days or three. Now she has fasted three days; what does she have to do?
He replied: If a person is not sure about how many days he has to make up, then he should go with the lower number. So if a woman or a man is not sure whether she or he has to make up three days or four, then he or she should go by the lower number, because the lower number is more certain and anything more than that is subject to doubt, and in principle, one does not owe any days (except those one is certain about). However, in order to be on the safe side he or she should make up this day about which there is some uncertainty, because if it was obligatory, then the obligation will have been fulfilled, and if it was not obligatory then it will be a voluntary action and Allah, may He be exalted, will not cause the reward of any good deed to be lost.
End quote from Fataawa Noor ‘ala al-Darb.
And Allah knows best.