You can post as many Add to Cart buttons for different products as you wish in any page anywhere; just remember that every page which has any Add to Cart buttons must also have at least one block of View Cart button code somewhere in the same page as well.
Typically, your buyer would Add to Cart each type of item they wish to purchase, then in their cart screen, they would edit the quantity they wish to order of each item and click Update Cart to see a fresh order total before proceeding to Checkout. This is fairly standard behavior for most shopping carts online, so anyone who has done online shopping before should already be familiar with this process. Note that to allow your buyers to do this, you will need to enable the "Let buyers edit quantity in cart" setting for each product in your Seller Admin.
