How do I text more than 20 at a time? Do I need another program on my Iphone

You can find general information about Group Messaging here:

Send a group text message on your iPhone or iPad - Apple Support

You can also create a Contact List (i.e., a Group), within your Contacts App, for the intended recipients.

Each member of the list must have a contact card within your Contacts App - obviously with the recipient email address.

To create your list, ensure the Contacts Sidebar is visible; tap the Sidebar button at top-left of the Contacts Window to expose the sidebar.

To create a new list, tap the Edit button at the top of the sidebar - then tap the Add List button at bottom-left. Give the list an appropriate name, then tap Done. Next tap the new list that you have created; this will open the list editor, where you can add contacts to your list with the Add Contacts button - adding additional contacts as required.

When you’ve finished, you can use the Contacts List(s) that you have created (by name) for Messaging.

Is this a group of friends, or is this a group of clients of a business? If for business, there are many policies in place that govern how and when you can send mass messages. You would need to work with an SMS provider like Attentive or Twilio to send messages to clients.