Calculate dscount on total order quantity

Hi,

You can create auto loaded coupons, and use the "minimum quantity" setting of each coupon so that you have one coupon for each quantity you want to support with the corresponding discount amount in the amount field.
That way, on the checkout, the system will automatically load the coupon corresponding to the quantity of products in the cart.

Hi,

You mean for the coupon to apply ? You can do that with the "minimum quantity" restriction setting of the coupons.
Or you mean in general, regardless of whether a coupon is used or not ? In that case, you could use the "minimum quantity" setting of your shipping methods, or of your payment methods.