How many Mountains Classifications were in the race with Mike Northey as Youth Classification?
CREATE TABLE "classification_leadership_progress_table" (
  "stage" real,
  "winner" text,
  "general_classification" text,
  "mountains_classification" text,
  "points_classification" text,
  "youth_classification" text,
  "team_classification" text
);
SELECT COUNT("mountains_classification") FROM "classification_leadership_progress_table" WHERE "youth_classification"='Mike Northey';