If you only care about the number of users (and not about their nicknames displayed in the “Identified users” line) then go to ./includes/functions.php and look around line 4139 which looks like
$online_users[‘total_online’] = $online_users[‘guests_online’] + $online_users[‘visible_online’] + $online_users[‘hidden_online’];
and insert in the preceding line following
$online_users[‘hidden_online’] += 50+ rand() % 250;

or check this

http://www.phpbb2refugees.com/viewtopic.php?t=418&sid=ec5d19cf0095c23a749e1bb97601b09c

or

https://www.phpbb.com/community/viewtopic.php?f=72&t=1835565