According to the eighth edition of "Mountaineering: The Freedom of the Hills" (2010) book there are ten essentials. They were given as a guideline, where you needed the group to have each of these areas covered. Not each person needs all 10 of these, but there should be a little redundancy in case the group splits into smaller teams or gets isolated.
The book also recommends adding extra equipment to handle your specific terrain, as well as signaling devices. They could include a whistle, mobile phone, two-way radio, satellite phone, unbreakable signal mirror or flare, laser pointer, or any items that could contact potential sources of assistance