This will display tickets sold / datetime limit. It is an example and currently just lists the data with no formatting. It should be added to the loop of an EE4 Custom Template

ee4_custom_template_attendee_count_example.php

if( $event instanceof EE_Event ) {
		
			$spaces_remaining = 0;

			$datetimes = $event->get_many_related( 'Datetime', array( 'order_by' => array( 'DTT_EVT_start' => 'ASC' ) ) );
			foreach ( $datetimes as $datetime ) {
				if ( $datetime instanceof EE_Datetime ) {
					$dtt_spaces_remaining = $datetime->spaces_remaining( TRUE );
					// if datetime has unlimited reg limit then the event can never be sold out
					if ( $dtt_spaces_remaining === INF ) {
						return FALSE;
					}
					else {
						$spaces_remaining = max( $dtt_spaces_remaining, $spaces_remaining );
								$max_spaces = $datetime->get_raw( 'DTT_reg_limit' );

					}
				}
			}

			echo $post->post_title . ' - ' . $spaces_remaining . ' / ' . $max_spaces . '<br>';


		}