The love and devotion that Ruth had for her mother-in-law says much about Naomi, as well as Ruth. Others noticed Ruth’s love for Naomi, and so they said to Naomi,
“And may he be to you a restorer of life and a nourisher of your old age; for your daughter-in-law, who loves you, who is better to you than seven sons, has borne him” (Ruth 4.15).
To heighten the value of Ruth to Naomi, they said that she was better than seven sons. Naomi had already lost her two sons to death, but now this devoted daughter-in-law gives Naomi another son, because Ruth considers Naomi a grandmother to this son, although technically she was not.
Seven here has significance, because these women wanted Naomi to know just how special Ruth was, and thus they used this perfect number to highlight the greatness of this woman.