The maximum number of characters in one text is 160 characters, including spaces.
So if your text is 160 characters or less, then 1 text credit will be used.
You can send messages that are longer than 160 characters but this will use more than one text credit.
You can send a long message of up to 612 characters which will use 4 text credits.
When sending long messages, they will appear on the recipients' mobile handsets as 1 long text.
When you write your message in MessageBox, you will be told how many text credits will be used to send the message.