One sms is 160 characters.
You can combine 3 sms messages before auto conversion, so that is 480 characters.
On an iPhone, you can go into settings and turn of MMS completely so you can safeguard against accidental sending.
Community help link here
https://community.o2.co.uk/t5/Other-Products-and-Services/Accidental-MMS-Sending-and-How-to-Avoid-it...
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