BuddyPress.org

BP doesn’t track who is logged-in.
It does track last_activity which is based on actually doing something on the site – not just being logged-in and having the site open in a browser.

So the best you can do is to show a number based on the number of members with last_activity within the last x minutes.

Try:

function boris_online_number( ) {
	global $wpdb;
	
	$time = date('Y-m-d H:i:s', time() - ( 15 * 60) );  // minutes * seconds
	
	$a_number = $wpdb->get_var( "SELECT COUNT(user_id) FROM {$wpdb->prefix}bp_activity WHERE component = 'members' AND date_recorded > '$time' ");
	
	echo 'Online: ' . $a_number;
}
add_action( 'bp_ready', 'boris_online_number' );