An SMS is made of 160 characters, according to international standards. In those 160 characters are recognized spaces, punctuation, letters, numbers and special characters. However, you are not restricted to those 160 characters, you can send longer messages, but it will be counted this way: 1 SMS every 160 characters.
Be careful when you use the SMS merge option –that is the personalization of your SMS with information on your contact file– be careful that the number of characters does not exceed 160 or it will double the price of your professional sms.
Example: You can start your message with the name of your client/patient, but beware that if that name is quite long, it may make you exceed 160 characters and therefore count as 2 SMS. But do not worry, the number of SMS will be indicated before the validation of the sending.