Event Tickets (ET): Limit each logged-in user to a maximum purchase quantity per event.

functions.php

<?php

/**
 * Event Tickets (ET): Limit each logged-in user to a maximum purchase quantity per event.
 *
 * !!! Change the maximum to your own value!!!
 * Requires ET version 4.11.1 or later, since that's when front-end ticket inputs fully supported this filter.
 *
 * @link https://gist.github.com/cliffordp/009f0b5f9cda516368545957eaf8543e This snippet.
 * @link https://docs.theeventscalendar.com/reference/hooks/tribe_tickets_get_ticket_max_purchase/ This filter.
 * @link https://tribe.uservoice.com/forums/195723-feature-ideas/suggestions/18616273-limit-rsvp-or-woocommerce-ticket-registration-to-o Feature request.
 *
 * @see  \Tribe__Tickets__Tickets_Handler::get_ticket_max_purchase() Where this filter fires.
 *
 * @param int                           $available Max purchase quantity.
 * @param Tribe__Tickets__Ticket_Object $ticket    Ticket object.
 * @param WP_Post                       $event     Event post.
 * @param int                           $ticket_id Raw ticket ID.
 *
 * @return int
 */
function cliff_event_tickets_max_qty_per_event_per_user( $available, $ticket, $event, $ticket_id ) {
	// TODO: Change this to what you want!
	$max_qty = 1;

	// You should require being logged in to purchase: wp-admin > Events > Settings > Tickets > "Login Requirements"
	$user_id = get_current_user_id();

	if ( empty( $user_id ) ) {
		return $available;
	}

	$attendees = tribe_attendees();

	// Enable found() calculations.
	$attendees->set_found_rows( true );

	// Do the filtering.
	$args = [
		'event' => $event->ID,
		'user'  => $user_id,
	];

	$attendees->by_args( $args );

	// Account for unlimited quantity
	if ( -1 === $available ) {
		$available = $max_qty;
	}

	// Do the overriding of the max allowed
	$max_qty = $max_qty - $attendees->found();

	$max_qty = min( $max_qty, $available );

	// Don't allow displaying a negative quantity available
	if ( 0 > $max_qty ) {
		$max_qty = 0;
	}

	return $max_qty;
}

add_filter( 'tribe_tickets_get_ticket_max_purchase', 'cliff_event_tickets_max_qty_per_event_per_user', 50, 4 );

@onisunset
Try commenting out 'event' => $event->ID, from the $args and if 0 !== $attendees->found(), then set the $max_qty = 0

If that doesn't work for you, please report back what did work for you, to help others. If you have follow-up questions, I cannot provide support here so please request your customization at https://support.theeventscalendar.com/

It worked, thank you!

Excellent! 💯