what bus number do i take going to hefei airport from the fast train staion?

Do you refer to the bullet train station? If yes, it's the South Railway Station.
You can take airport shuttle bus line 3 at there directly. In general, it takes around an hour on the way. If by taxi, the duration is about 30 to 40 mins. It costs CNY 130 to 140 in the day time.
In addition, the airport has only one terminal building. Don't worry.