ما معنى الليالي ذوات العدد؟

The Prophet, peace and blessings be upon him, used to seclude himself in the Cave of Hira for many nights for tahannuth (worship). Then he would return to his family and provision himself for a similar number of nights. Those nights, "of a certain number," were the periods he spent there. Their number was left unstated due to variation, and it is probable that they were many.