Hi,
You can do that with the Essential or the Business editions by creating a coupon via the menu Orders>Coupons.
There, you can restrict the coupon by category (which you can't do with the Starter edition as the coupon settings are limited), enter a minimum number of products, and turn on the "auto load in cart if possible" setting of the coupon.
That way, the discount amount of the coupon will be applied automatically to the cart if there are in the cart at last the number of products you entered in the coupon for the category selected in the coupon.