Questions answered by this recipe
How can I know how many visitors are currently surfing on my site?
In your wiki pages, add the markup (:CurrentVisitors:) to display the number of visitors (strictly speaking "different IP addresses") that came to your site during the last MaximumNumberOfSecondsToConsiderAVisitorAsCurrent.
The counter will update only when action=browse.
Added the following variables:
(:MaxCurrentVisitors:) displays the maximum number of visitors online at the same time.
(:MaxCurrentVisitorsTime:) displays the time when the maximum of visitors was reached.
See discussion at CurrentVisitors-Talk