Hello 
I am looking into this option too this is very important, so we can set it from price lists to specify per customer.

and this is the difference between Buy X and get Y, because in this case you will be Buy X number and get X number for free.

Hi, to configure this requirement, you can use the "Discount & Loyalty" option in odoo, to enable this:              Go to the Settings of Sales App under the configuration menu and enable the "Discounts, Loyalty & Gift Card" option. After this, a new menu ("Discount & Loyalty") will be added under the Products menu,               go there and create a new configuration, choose the "Program Type" as "Buy x Get y" and set the necessary rules and rewards under it.

Hope it helps