The best I could find was the Roman Ritual of 1964 which did not differ substantially from the Roman Missals of 1952 or 1962. Accordingly it states:
Anointing of the sick can be given only to one of the faithful–one who has attained the age of reason and who is in danger of death by reason of illness or old age. The sacrament may not be given more than once during the same illness, unless after receiving the sacrament, the sick person has recovered from the danger and then has a critical relapse.
Prior to Vatican II the sacrament would have been received once by someone in danger of death, but if they recovered and then become in danger of death again they would have been anointed again.
According to the Catechism of the Catholic Church today the sacrament may be received more than once under the following conditions:
1515 If a sick person who received this anointing recovers his health, he can in the case of another grave illness receive this sacrament again. If during the same illness the person’s condition becomes more serious, the sacrament may be repeated. It is fitting to receive the Anointing of the Sick just prior to a serious operation. The same holds for the elderly whose frailty becomes more pronounced.