The Home Office do not hold central records of the number of individuals in Immigration detention that are referred for, or that self-refer for Rule 35 assessments.
Medical records for individuals in Immigration detention are confidential and are not routinely shared with the Home Office unless the individual has consented to share this information. This includes appointments and assessments with a medical practitioner. Notification of Rule 35 assessments, which are carried out by Doctors, are therefore not routinely shared with the Home Office.
The Home Office do however, centrally record the number of Rule 35 reports raised by Doctors under the Detention Centre Rules 2001. This information is published quarterly in Immigration Enforcement Transparency data found at https://www.gov.uk/government/publications/immigration-enforcement-data-november-2018.