How do I delete a number from a group text message

Add or remove someone from a group iMessage conversation

Anyone in the group message can add or remove someone from the conversation. To add a person to a group message, tap Details, then tap Add Contact.

You can remove a person from a group message that has at least four people. Tap Details, then swipe from right to left on the name of the person you want to remove. Then tap Delete.

from > Send a group message with your iPhone, iPad, or iPod touch - Apple Support