The recommended maximum number of parts of a concatenated message is six. Sinch does not recommend sending messages with more than six parts as there might be a limitation at the respective service providers or on the handset where the messages are being terminated.
Please, take a look at the article "What is a Concatenated SMS?", to learn more about concatenated messages and at the "Special Character Considerations for SMS Messaging" article, to see how special characters can affect the length of your messages.
