Buy x Amount Get x Number Free WooCommerce Full tutorial and support @ https://wpsites.net/?p=116705

buy-x-get-number-free.php

add_action('woocommerce_cart_calculate_fees', 'buy_ten_get_two_free', 10, 1 );
function buy_ten_get_two_free( $cart ){
    if ( is_admin() && ! defined( 'DOING_AJAX' ) ) return;

    // Set HERE your targeted variable products IDs
    $targeted_product_ids = array( 185 );

    $each_n_items = 10; // Number of items required to get a free one
    $free_items_count = 2; // Number of free items to give
    $discount = 0; // Initializing
    $count_items = 0; // Initializing counter for eligible items

    foreach ( $cart->get_cart() as $cart_item ) {
        if( in_array( $cart_item['product_id'], $targeted_product_ids ) ) {
            $qty = intval( $cart_item['quantity'] );
            $count_items += $qty;
        }
    }

    // Calculate number of free items
    $free_items = floor( $count_items / $each_n_items ) * $free_items_count;

    // Calculate total price of items eligible for discount
    foreach ( $cart->get_cart() as $cart_item ) {
        if( in_array( $cart_item['product_id'], $targeted_product_ids ) ) {
            $qty = intval( $cart_item['quantity'] );
            $price = floatval( $cart_item['data']->get_price() );

            // Calculate how many items are eligible for discount
            $eligible_qty = min( $qty, $free_items );

            // Calculate the discount based on eligible quantity
            $discount += $eligible_qty * $price;

            // Deduct the eligible items from free_items count
            $free_items -= $eligible_qty;
        }
    }

    if ( $discount > 0 ) {
        // Displaying a custom notice (optional)
        wc_clear_notices();
        wc_add_notice( __("Buy 10 Get 2 Free"), 'notice');

        // Apply the discount
        $cart->add_fee( __("Buy 10 Get 2 Free"), -$discount, true  );
    }
}