Yes, you apply RTI to know about the Number of Vacancies of Judges in Any Court.
Courts are functionaries of the Judicial System and are "Public Authorities" as per Section 2 (h) of the RTI Act 2005. You can file the RTI application to the concerned High Courts to know about its Vacancies and of Lower / Sub-ordinate Courts under the High Court. You can also file RTI to Department of Justice, Ministry of Law and Justice for vacancies of Judges in the Supreme Court and concerned High Courts.
You can file the RTI Application to the designated "Public Information Officer (PIO)" by making the payment of the prescribed fee.
To know more about the procedure to file RTI Applications, please refer https://onlinerti.com/about-rti
You can also file the RTI application to Department of Justice, Ministry of Law and Justice either Online or Offline. To know more about the procedure to file RTI Applications Online for Central Government Departments, please refer https://onlinerti.com/how-to-file-rti-online-for-center