Fosamax technically isn't a bone builder. With severe osteoporosis and a history of fracture, I would be scared to rely on prunes, but it would be helpful info as an addition to bone building meds!

This study (Funded by the Prune Boards!) says 5-6 prunes a day
https://www.medicalnewstoday.com/articles/eating-5-to-6-prunes-a-day-may-prevent-bone-loss-osteoporosis

“Our study is the first to reveal that consuming just 5-6 prunes a day prevented a loss of bone mineral density at the hip, the site of most concern for aging men and women as hip fractures commonly lead to hospitalization, diminished quality of life, and loss of independence.”
— Dr. Mary Jane De Souza