Do i need a separate bar code for every product?

Yes. You also need a different code for every variation of a product (e.g. size, color). For instance, if you sell a shirt that comes in three sizes you need three different bar codes. That way, if one size starts to sell faster than the others, the store can tell what needs to be restocked without having to do a manual inventory count. If a shirt comes in three different colors as well as three different sizes, you’ll need nine separate bar code numbers.

Another example is if you have a large amount of a single type of item, like bottled water, you can have a million 12 oz. bottles of water and only need 1 bar code to print on all of them. 

Each bar code stays with the assigned item for life and when you cease manufacturing that item you would cease using that unique bar code number. This is done so that confusion and conflicting data is kept to zero. Also many store's with online P.O.S. systems remember the codes used on a specific product for the lifetime of the site and do not let you use the bar code on a different item.