You shouldn't need another wrench unless the lug nuts arent the factory originals. You can easily get another wrench at an autoparts store, or another set of lugs. Either way is fine. Just be sure that they match.
you mean 21mm not 20mmThey use a standard 20mm, 6 or 12 point socket or equivalent. But if you have wheel locks, one nut on each wheel will require the key to go between the 20 mm socket and the nut.