How to send a group text to more than 10 people?

Solution 1:

In the iPhone you'll need to make a group in your contacts list then try to send them all a message and see if it allows you to send that message to all members of the group. By default that should work.

If not, then there are serval third-party apps that can keep an unlimited about of members in them for a group chat and they work on multiple smartphone software platforms:

WhatsApp, FaceBook Messenger, Telegram, GroupMe, Kik, Google Hangouts, Google Allo (if it is still around), Skype, Viber, LINE, WeChat, Discord (has channels and rooms for deep integration), Slack (perfect for business teams working on solo or group projects), BBM, Voxer, Tango, Snapchat, Signal & many more.

With GroupMe not everyone needs to have the app installed. You can install the app, and send the text out to 20 people and any one of those 20 can just reply back to your text and it gets sent to all 20. Only if they want to initiate a new group text will they need the GroupMe app.

Solution 2:

A group message can be sent either as an iMessage or an SMS. Just create the group to which you wish the message, the Messages app will automatically choose the appropriate message type based on recipients' settings, network connection, and carrier plan.

In depth guide on sending group message can be found in the Apple Support Document, Send a group message on your iPhone, iPad, or iPod touch.