List the number of builders where the withdrawn is 1951 and number is 42.
CREATE TABLE "collection" (
  "date" text,
  "builder" text,
  "type" text,
  "operator" text,
  "number" text,
  "withdrawn" text,
  "status" text
);
SELECT COUNT("builder") FROM "collection" WHERE "withdrawn"='1951' AND "number"='42';