how do I disable new user registration email for specific admin user?

Going by wordpress wp_new_user_notification, github, wordpress code reference, the function that handles sending email for new users, it only send email to user and site admin by default.

There’s a filter wp_new_user_notification_email_admin that you can hook into to change the admin details, maybe you have a plugin that uses this filter.

Try disabling them one by one and see if it fixes it.