Each recipient who is sent your message counts as 1 message.
For example, a message sent to 200 recipients would count as 200 messages.
(Note that if your message is more than 160 characters, it will split into 2 or more messages so it will use up more messages on your account.)
Sending as MMS
You also have the option to send as an MMS message. This option allows you to include an image as well as have up to 1550 characters in your message, but it counts as 3 messages per recipient instead of 1. (So with this option, a message sent to 200 recipients would count as 600 messages, for example.)