How to reward customer a gift if he Buy 2 items of the same product? (Buy 2X get Y offer)

If you want to create a Buy 2X/3X/nX to get Y offer (Ex: Buy 2 books get a free pen), you can enable the “All products in the cart must be the same” feature and insert the number of X customer have to buy in the “Min of total Cart’s quantity” field.