What is the ruling on a group of people gathering to recite a specific surah a certain number of times with the intention of achieving something, and is there a specific surah or supplication for the release of a detainee?

It is not prescribed to read a specific Surah by a specific number of people and repeat it a certain number of times for the purpose of fulfilling a need, because it has not been mentioned in the Sharia. The Messenger of Allah (peace and blessings be upon him) said: "Whoever introduces into this matter of ours that which is not part of it, will have it rejected." The Sharia has indeed prescribed reading specific verses or Surahs at specific times for a particular benefit, such as Ayat al-Kursy before sleeping and the Mu'awwidhat for ruqyah, but it has not been narrated that they should be read by a group and for a specific number.

There is no authentic evidence for a specific Surah or supplication to release a prisoner or captive. Weak Ahadith have been reported concerning this, including a Hadith narrated by Al-Fudayl about reading "I put my trust in the Ever-Living, who does not die, and praise be to Allah, who has not taken a son...", and a Hadith by Malik Al-Ashja'i about frequently saying "La hawla wa la quwwata illa billah" (There is no power nor strength except with Allah) to release a captive.

It is permissible for a Muslim to frequently recite general remembrances (adhkar) that are said during distress, until Allah relieves their distress.