If a longer message (multi-part message) is sent, each message has a maximum of 153 characters due to seven characters being needed to facilitate re-joining of the message on delivery. For longer messages sent, the SMS is split into two or more messages that may be reassembled on delivery, so they are a single message (or, on some handsets, may be delivered as a series of separate SMS).
If you send an SMS text message that contains more than 160 characters or 1 Message Parts, every 153 characters or part thereof will be charged as an additional SMS message.