SELECT COUNT("Milepost") FROM table_42484 WHERE "Name of the tunnel" = 'twin tunnel #1 north'
What is the Milepost of the Twin Tunnel #1 North?
CREATE TABLE table_42484 (
    "Milepost" real,
    "Name of the tunnel" text,
    "Length" text,
    "Maximum Height" text,
    "Minimum Height" text
)