As I am currently in the process of rewriting my company's Gender Transition Guidelines, I can say that at least your immediate supervisor, HR, and anyone who you interact with on a regular basis will likely need to know. However, you should only have to personally inform HR and/or your supervisor of your intent to transition. I do not know what type of business you are in or what your position is at your company, but those would be people who would likely need to know of your transition. Ask your HR Dept if they have any Gender Transition Guidelines type of documents.