A single message can accommodate up to 1600 characters. However, the cost of an SMS is calculated based on 160 characters per SMS, which means if you send a single message of 1600 characters, it will be equivalent to 10 messages, cost-wise.
The SMS feature is in the Bronze plan and the charges are different for different countries.
To know the SMS charges of different countries:- https://callhippo.com/downloads/SMS-rates_countrywise.xlsx