Build a list of user IDs, ordered by the number of known (WooCommerce) subscriptions each user is associated with.

users-by-total-subscriptions.sql

SELECT   meta_value AS user_id, 
         COUNT(*) AS subscription_count
FROM     wp_posts
JOIN     wp_postmeta ON ID = post_id
WHERE    post_type = 'shop_subscription'
             AND meta_key = '_customer_user'
GROUP BY meta_value
ORDER BY subscription_count DESC