CiviDiscount - Automatic discounts for number of attendees
this can be achieved using an event price set. If you create a price set you can create a ticket option which takes up multiple "seats" at the event. You can create a 10 person ticket option and then set the price of that option.

Note however in that approach you cannot require that all 10 attendees submit separate registration details and you can only collect the details from the first participant who is buying the 10 person ticket.

Best