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?
CREATE TABLE "head_quarters_and_support_regiments" (
  "unit" text,
  "complement" text,
  "killed" text,
  "wounded" text,
  "missing" text
);
SELECT "missing" FROM "head_quarters_and_support_regiments" WHERE "killed"='0 off 0 men' AND "wounded"='0 off 0 men' AND "unit"='royal waggon train';