Hi,
It depends how you configured your shipping methods.
If you set the shipping price for each product (because the have the "price per product" option of your manual shipping methods activated), when you add the shipping price for each product, you can add several prices based on the quantity of the product in the cart.
Another possibility in the manual shipping methods listing is to create several manual shipping methods, one for each range of number of products in the cart (if all the products have the same weight, you can use the weight restriction of your manual shipping methods for the range) and set its price accordingly.