Each outgoing message uses credits. All incoming messages are free.
The length of your text and the characters you include determine how many credits are used. Texts can be up to 765 characters in length. However, if your text includes an emoji or a special character (û, á, etc.) your text will be limited to 335 characters.
A 160 character text with no special characters sent to 500 people = 500 credits.
20 people reply back with questions = 0 credits.
***MMS (picture messages) will use 3 credits per recipient regardless of how many characters you use or if you include special characters.