How large a sample do I need?

#1
I want to test a postcard. I have a list of 15,000 names, 4,000 of whom have already signed up for information. I want to mail an offer to some statistically significant sample of the remaining 11,000. How many do I need to mail?
 
#2
Additional information needed

In order to help you with your quetion we need some additional information, such as what it is you are testing. If you are conducting a survey what type of information are you going to get (dichotomous vs polytomous). Also expected response rates etc.