Prunes are a very effective treatment for constipation for many people. They are a fruit, but being dried, are higher in sugar than many others. Three to four prunes a day seems reasonable to me, and is effective for many people to treat constipation.
A recent review of studies on the effects of prune eating on bone mineral density suggested that there is some benefit. However, the studies were neither consistent nor of high-enough quality that I would recommend prunes solely for their purported benefit on bones. Moreover, most of the studies used about 100 grams of prunes per day: 10-12 average-size prunes. source
Constipation Causes and Treatment