The maximum number of volumes that can be attached to a single instance is currently limited to 26. This solution allows the customer to configure the maximum number of volumes that can be attached to a single instance, allowing them to attach more than 26 volumes.
Content available on the Customer Portal: https://access.redhat.com/documentation/en-us/red_hat_openstack_platform/15/html-single/advanced_overcloud_customization/index#sect-Config_max_disk_devices_to_attach