Set default quantity of 1 and READONLY for all tickets: Same as https://gist.github.com/cliffordp/5b57df71be8b52f595817ddbf81acdab except make quantity readonly

functions.php

<?php
/**
  * Set default quantity of 1 and READONLY for all tickets:
  * - Event Tickets RSVP
  * - Event Tickets Plus WooCommerce
  * - Event Tickets Plus Easy Digital Downloads
  *
  * From https://gist.github.com/cliffordp/80b33455779b74ec49f6ea3033cb47bf
  *
  * Same as https://gist.github.com/cliffordp/5b57df71be8b52f595817ddbf81acdab except make quantity readonly
  * Watch out if you have more than 1 ticket per event!
  *
  * ! You may want to use https://gist.github.com/cliffordp/9a457b724e38b3036f8d48adc90930ed instead!
  */
function cliff_all_tickets_default_quantity_and_readonly() {
	// bail if not on a Single Event page
	if ( ! function_exists( 'tribe_is_event' ) || ! tribe_is_event() ) {
		return false;
	}
	
	wp_enqueue_script( 'jquery' );
	?>
	<script type="text/javascript">
		jQuery(document).ready( function () {
			// RSVP, Woo, and EDD tickets default to quantity of 1
			jQuery( 'input.tribe-ticket-quantity, .woocommerce .quantity input.qty, .edd.quantity input.edd-input' ).val( 1 ).attr( 'readonly', true );
			
			// CSS to display RSVP tickets' "Send RSVP confirmation to" fields
			// Note: will continue to show even if user changes quantity to zero because we didn't bind to the field to continually watch it
			jQuery( 'tr.tribe-tickets-meta-row' ).show();
		});
	</script>
	<?php
}
add_action( 'wp_footer', 'cliff_all_tickets_default_quantity_and_readonly' );

for single ticket per event you can change the script part to:

<script type="text/javascript"> jQuery(document).ready( function () { //when field changes jQuery('input.tribe-ticket-quantity').on('keyup', function() { var changedField = this; jQuery('input.tribe-ticket-quantity').each(function(){ //change all fields to 0 this.value=0; }); // and set changed field to 1 changedField.value = 1; }); }); </script>

@daliakopoulos, you can change it however you like to customize it further. If you're needing additional support for this snippet or are reporting it broken, please contact Event Tickets Plus support at https://support.theeventscalendar.com/submit_ticket

input.tribe-ticket-quantity should be input.tribe-tickets-quantity. not sure if this was a change in the class name on the field, or a typo