If you are going by the WPATH guidelines, you need one letter for hormones and two for surgery. The letters should be written by a "qualified mental health professional", basically a psychologist with a Masters or Phd, or a psychiatrist.
If you bypass the WPATH guidelines and go "informed consent", then you probably don't need letters at all. It may be up to your insurance to decide that. You should check if they have any requirements.
Some doctors and agencies layer additional requirements on top of the WPATH guidelines. Locally, that makes getting your letters a mess. Dr. Brassard, for example, apparently wants one of the letters to come from a Phd level psychologist or a Psychiatrist. And our local health care system requires one of the letters to be from a "specialist-level" medical doctor. So to meet the combined requirements in these parts, you have to choose your referees carefully: a psychiatrist would fill several of the requirements, so only one other letter would be required. But if your specialist is an endocrinologist, then you would need three letters. I give you the example from this area only to show the need to check with your insurance, and with the doctor you intend to see, to see what they require.
