Handle item limit for custom coupon types

Fixes #20455

includes/class-wc-discounts.php

@@ -499,11 +499,28 @@ protected function apply_coupon_fixed_cart( $coupon, $items_to_apply, $amount =
 	 * @return int Total discounted.
 	 */
 	protected function apply_coupon_custom( $coupon, $items_to_apply ) {
+		$limit_usage_qty = 0;
+		$applied_count   = 0;
+
+		if ( null !== $coupon->get_limit_usage_to_x_items() ) {
+			$limit_usage_qty = $coupon->get_limit_usage_to_x_items();
+		}
+
+		// Apply the coupon to each item.
 		foreach ( $items_to_apply as $item ) {
-			$discounted_price  = $this->get_discounted_price_in_cents( $item );
+			// Find out how much price is available to discount for the item.
+			$discounted_price = $this->get_discounted_price_in_cents( $item );
+
+			// Get the price we actually want to discount, based on settings.
 			$price_to_discount = wc_remove_number_precision( ( 'yes' === get_option( 'woocommerce_calc_discounts_sequentially', 'no' ) ) ? $discounted_price : $item->price );
-			$discount          = wc_add_number_precision( $coupon->get_discount_amount( $price_to_discount / $item->quantity, $item->object, true ) ) * $item->quantity;
-			$discount          = min( $discounted_price, $discount );
+
+			// See how many and what price to apply to.
+			$apply_quantity = $limit_usage_qty && ( $limit_usage_qty - $applied_count ) < $item->quantity ? $limit_usage_qty - $applied_count : $item->quantity;
+			$apply_quantity = max( 0, apply_filters( 'woocommerce_coupon_get_apply_quantity', $apply_quantity, $item, $coupon, $this ) );
+
+			// Run coupon calculations.
+			$discount = wc_add_number_precision( $coupon->get_discount_amount( $price_to_discount / $item->quantity, $item->object, true ) ) * $apply_quantity;
+			$discount = wc_round_discount( min( $discounted_price, $discount ), 0 );
 
 			// Store code and discount amount per item.
 			$this->discounts[ $coupon->get_code() ][ $item->key ] += $discount;