How many Biometric Invitation letters are sent by UKBA?

I have one weird query but it is just out of panic that I am in to.

My application is pending with UKBA since March and I have not received the biometric invitation letter yet.
Somehow I contacted them and they told me that "I can confirm that biometric letters are sent to your correspondence address".

My application has 2 dependents as well. I want to know is there a single Biometric letter sent for complete family or one letter per applicant in the application.

Very worried so want to understand the format of biometric letter. Don't want to keep pestering UKBA with such questions before getting help from senior members here.

Biometric letters are sent per person (I think)

You will get three biometric letters, one for each applicant and this would come in a single envelope.

Thanks for calrifying my query.
So I think now my application is on track as UKBA said that they have sent letters to my address.

Keeping fingers crossed as they say "There are many slips between cup & the lips."