The Events Calendar (TEC) and Event Tickets (ET): Get the total number of unique events for which a user has obtained tickets (of any kind, including RSVP) within the specified Event Category.

functions.php

<?php
/**
 * The Events Calendar (TEC) and Event Tickets (ET): Get the total number of unique events for which a user has
 * obtained tickets (of any kind, including RSVP) within the specified Event Category.
 *
 * Could be turned into a shortcode. Could be used in a way to output a friendly message like,
 * "Thanks for attending *14* events in the *Sports* category."
 * This function currently gives you the *14* part of this message but could easily be extended.
 * 
 * @link https://gist.github.com/cliffordp/ddb9e11015c193afd1eea78a54afdf79 This snippet.
 *
 * @see tribe_events()
 * @see tribe_attendees()
 *
 * @param int $tec_cat_id
 * @param int $user_id If not specified, will use the currently logged in user.
 *
 * @return int|false
 */
function cliff_et_get_count_of_events_user_obtained_ticket_for( $tec_cat_id, $user_id = 0 ) {
	// Bail if bad request.
	if (
		! function_exists( 'tribe_events' )
		|| ! function_exists( 'tribe_attendees' )
		|| ! is_int( $user_id )
		|| ! is_int( $tec_cat_id )
	) {
		return false;
	}

	// If no User ID passed, get currently logged in user.
	if ( 0 === $user_id ) {
		$user_id = get_current_user_id();
	}

	/**
	 * Get all the event IDs that are in the specified event category that also have a ticket assigned (to help reduce
	 * the foreach looping).
	 *
	 * @see Tribe__Events__Repositories__Event
	 */
	$events = tribe_events();

	$events->by_args(
		[
			'event_category'    => $tec_cat_id,
			'cost_greater_than' => - 1, // has any type of ticket (even free or RSVP)
		]
	);

	$event_ids = $events->get_ids();

	$user_event_attendance_count = 0;

	foreach ( $event_ids as $event_id ) {
		/**
		 * Get all the attendees for this user (and optionally for a specific set of events).
		 *
		 * @see Tribe__Tickets__Attendee_Repository
		 * @see Tribe__Tickets_Plus__Attendee_Repository
		 */
		$attendees = tribe_attendees();

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

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

		$attendees->by_args( $args );

		if ( 0 < $attendees->found() ) {
			$user_event_attendance_count ++;
		}
	}

	return $user_event_attendance_count;
}