Here is a sample template you can use to write a letter acknowledging a subject access request.
To [Recipient name]
Dear, [Recipient name]
This letter is to acknowledge your subject access request under Article [Number] of the [Policy name].
In order to process your subject access request, we will require identity confirmation in the form of [Identification document]. This will be appreciated as it will speed along the entire process.
Additionally, we request you to give a detailed account of the information that you require such as the name of the department, information for a particular service and other such queries. A period of 1 month is provided to process your request once proof of identification is supplied.