You could accomplish this 2 ways:
Option 1: You would set up seperate skus for your offers buy one get one, two, three ... etc. In the product setup screen there is a field called "items in sku" that allows you to tell the cart how many individual items are in the sku.
Items in Sku – Items in Sku is used if you are bundling multiple products in a single sku. For example, a “But 2 Get 1 Free Deal”. It is used in conjunction with the coupon feature giving you the ability to control the coupon discount. For example, if you are running a promotion for 5 dollars off and individual products and the customer purchases the buy 2 get 1 free you can give them 10 dollars off. This feature allows you to use a single coupon codes for the single items or the bundled items.
Option 2: You could setup a bundled product using the "use primary price and weight" checkbox.