First, this help page explains our shipping calculation settings:
You could just set up a shipping rule for United States using the "Price per unit weight" method and set the rate for that to 1.00, so then you could configure Shipping Options for each product with a Weight equal to the per-unit shipping cost -- e.g., if you set a rate of 1.00, then a product with a unit weight of 5.00 would add 5.00 (= 1.00 X 5.00) to the cart's shipping charges.
If you want to charge more for International shipments, you could set up that rule for All Countries with a "Price per unit weight" rate as a multiple of the domestic rate -- e.g., if you set this rate as 1.50, Int'l shipping charges would be 50% higher than domestic, or 2.00 would double the domestic charges, etc.
While you can define shipping rules with min/max item quantity thresholds, if you simply want all orders for at least X number of items to have free shipping, it would probably be easier to just create an automatic (no code) Discount for All Cart Items that takes 100% off the Shipping total when Min. Item Qty. = X.