If you are only selling tickets on your site, then you could go into Seller Admin > Cart Discounts and create a Cart Discount with no Discount Code (so the discount will apply itself automatically), set to take an appropriate percentage off the Item Total (e.g. $2 off $10 tickets would be 20% off) when the Min. Items In Cart is 10.
If you are selling other products, then you would need to create an Item Discount in your ticket product's settings, with a code that buyers could enter in their cart for $2 off the per-unit price of that product, but you'd have to trust buyers to not try entering the code if they're ordering less than 10 (and refund payments or deal with people personally who can't follow directions).
