If you're using Feedburner already just sign into your account and the information is right there on your dashboard. If you're not using Feedburner sign up, generate a feed icon/email subscription box and paste the code into your template.
when you open your feedburner account " in google " you will see My Feeds , beside the FEED TITLE you will found the number of SUBSCRIBERS or as CountryBoy said f you don't have account with them .