Discount for products from the same category

Is it possible to set different prices (or discounts) if costumers buy several products (5 or more, for example) from the same category?

Thanks!

Not within the default OpenCart system, but Category & Product-Based Fee/Discount can do this.

Note that it's an Order Total extension, though. Order Totals do not affect the price shown in the cart -- only "Special" prices can do that. Order Totals are automatically activated when the criteria are met, and show up as a line item on the order. If you want to inform the customer of the possible fee or discount, you'd need to do so some other way, such as a banner or HTML module.