How can you provide a volume discount on event tickets?
The hook hook_civicrm_eventDiscount runs at the right place where you could apply discounts based on the number of people in the booking ... but there is a caution in the documentation: "This hook is outdated - notable, CiviDiscount does not make use of it."  Maybe because of that there is no guidance on how to use it.  

You probably want to investigate the implications of it being 'outdated' before you choose to use this and it would be useful if someone could weigh in with some background on the comment - and ideally update the dev docs too!