Group Text does not apply any limitation to the number of recipients in group text. But you should be aware that sending messages to large groups might be a heavy task depending on your device and iOS version.
So no worries when you are sending to a large group.
Kindly first confirm the warning message popup informing you the process might take some time by taping the SEND button:
You should be patient and wait until the message window appears. On some devices the screen could become black or freeze during the process, but at the end the window will appear.