how do i add more than 10 people to a text message?

Is this a one time deal or are you looking to make a group?

If it is one time - one just adds the names with a comma or semi colon between them

If you want to create groups

Log into iCloud.com

Go to contacts - click on the + sign in the left bottom and create groups

By getting an app designed for group messaging. Some that I've used include Telegram and GroupMe.