ET+ > Show Attendee Registration Field for first attendee only.

show-for-first-attendee-only.php

<?php
/**
 * Show Attendee Registration Field for first attendee only.
 * (Hide field for subsequent attendees.)
 * Can handle multiple fields.
 *
 * Usage: Add the snippet to your functions.php file or with a plugin like Code Snippets.
 *
 * @author: Andras Guseo
 *
 * Plugins required: Event Tickets Plus
 * Created: January 30, 2024
 */
function et_hide_fields() {
	// Bail if we don't have Event Tickets Plus
	if ( ! class_exists( 'Tribe__Tickets_Plus__Main' ) ) {
		return;
	}

	// Generic functions.
	if (
		(
			is_single()
			&& get_post_type() == 'tribe_events'
		)
		|| tribe( 'Tribe__Tickets__Attendee_Registration__Template' )->is_on_ar_page()
	) {
		?>
		<script>
            function fieldLabels() {
                /**
                 * ADD THE LABELS HERE
                 * Add the labels of the fields, which you only want to show for the first attendee.
                 * Only letters, numbers, and space.
                 */
                const labels = [
                    'Show for first only',
                    'Another Label',
                    'Yet Another Label'
                ];
                return labels.map(label => label.toLowerCase().replaceAll(" ", "-"));
            }

            function selectors() {
                return fieldLabels().map(label => "label[for*='" + label + "']");
            }

            function waitForElement(selector, callback) {
                let element;
                const intervalID = setInterval(function () {
                    element = document.querySelectorAll(selector);
                    if (element[0]) {
                        clearInterval(intervalID);
                        callback();
                    }
                }, 500); // Check every 500 milliseconds
            }

            function hideEm(fields) {
                fields.forEach(
                    function (item, index) {
                        // Skip the first attendee
                        if (index < fieldLabels().length) {
                            return;
                        }
                        // Hide the field.
                        item.parentElement.style.display = 'none';
                    }
                );
            }
		</script><?php
	}

	// Run on Single Event pages.
	if (
		is_single()
		&& get_post_type() == 'tribe_events'
	) {
		?>
		<script>
            // Wait for the ticket modal to appear.
            waitForElement('div.tribe-modal__wrapper--ar', hideFields);

            function hideFields() {
                // Listen for button clicks and field change.
                document.querySelector('.tribe-modal__wrapper--ar .tribe-tickets__tickets-item-quantity-number-input').addEventListener("change", hideFields);
                document.querySelector('.tribe-modal__wrapper--ar .tribe-tickets__tickets-item-quantity-add').addEventListener("click", hideFields);
                document.querySelector('.tribe-modal__wrapper--ar .tribe-tickets__tickets-item-quantity-remove').addEventListener("click", hideFields);

                const fields = document.querySelectorAll(selectors().join(', '));
                hideEm(fields);
            }
		</script>
		<?php
	}

	// Run on Attendee Registration page
	if ( tribe( 'Tribe__Tickets__Attendee_Registration__Template' )->is_on_ar_page() ) {
		?>
		<script>
            waitForElement('.tribe-tickets__form.tribe-tickets__attendee-tickets-item', hideOnAr);

            function hideOnAr() {
                const fields2 = document.querySelectorAll(selectors().join(', '));
                hideEm(fields2);
            }
		</script>
		<?php
	}
}

add_action( 'wp_footer', 'et_hide_fields' );

Maybe, to set fields required, change the following.

function selectors() {
                return fieldLabels().map(label => "input[name*='" + label + "']");
            }

and

function hideEm(fields) {
                fields.forEach(
                    function (item, index) {
                        // Skip the first attendee
                        if (index < fieldLabels().length) {
                            item.setAttribute( 'required', 'required' );
                            item.setAttribute( 'aria-required', 'true' );
                            item.setAttribute( 'placeholder', 'required' );
                            return;
                        }
                        // Hide the field.
                        item.parentElement.parentElement.style.display = 'none';
                    }
                );
            }

It might need some further tweaking though.