Table schema:
CREATE TABLE table_name_89 (missing VARCHAR, unit VARCHAR, killed VARCHAR, wounded VARCHAR)

Question: How many of the Royal Waggon Train Unit were Missing while having a Killed of 0 off 0 men and a Wounded of 0 off 0 men?
SQL: SELECT missing FROM table_name_89 WHERE killed = "0 off 0 men" AND wounded = "0 off 0 men" AND unit = "royal waggon train"