The character limit on an SMS message is 160 characters. If your message is above 160 characters it will split the text into multiple segments which will count as multiple messages. Under your company settings you can see the amount of messages you've sent VS the amount of message segments that were sent. The only way to avoid splitting messages into multiple segments is to limit the number characters in the SMS to 160 or below.
If you have any further questions regarding this, please feel free to send us an email at firstname.lastname@example.org.