SELECT
  customer_id
  , launch_at
  , count(is_first_occurence) over (ORDER BY launch_at) preceeding_customer_count
FROM
  (
    SELECT
      customer_id
      , launch_at
      , (CASE WHEN row_number() over ( PARTITION BY customer_id ORDER BY launch_at) = 1 THEN 1 ELSE NULL END) AS is_first_occurence
    FROM
    [table_with_launches]
  )