How many visas adoptive parents were allowed for international adoptions?

How can I find out how many visas adoptive parents were allowed for international adoptions in the year 1972 and did that number change by 1978? I'm writing a book and need some accurate information on the number of visas adoptive parents were allowed for international adoptions during the years 1972-78 . My recolleciton is that in 1972 parents were allowed only one visa, (exceptions were made for siblings) AND if the infant/child died before the child entered the US or before the adoption process was completed, the parents could not get another visa for another child.  I know that by 1978, the one-visa limit was lifted. (don't know if the Vietnam Baby Lift crash Apr 4, 1975 had anything to do with that one-visa limit being eliminated but I vaguely remember there still was the one-visa limit at the time.) There were, of course, adoptions by celebrities that somehow got around the rules, but I'm looking for data the average applicant would have had to abide by.  So, was there a one-visa limit in 1972? If so when was the one-visa limit voided?