You’d need to store the email addresses somewhere in the back end or the code, and use your code to determine which one/ones to send to when the form is submitted.

That is a pretty vague answer, and that’s becuase you’ve asked a pretty vague question. It’s hard ot give any sort of real coding advice unless we know how you’re trying to work things already.

Normally you’d set up your own admin page, or you can put in on the admin page of some other section in your site. The Settings API can help with that.

Then when your form is submitted you can use the values from that (assuming that you’re getting the values through POST here, of course).

if ($_POST ['form_setting'] == 'option_a') {
    $recipients = get_option ('option_a_emails');
}
else {
    $recipients = get_option ('option_b_emails');
}

That’s a very braod overview of how you could possibly do it.