You just made one...for every response you get from another person, that is considered a "ping".
No, that's not what a pinglist is. A pinglist is where you post articles/thread of a certain topic. People ask to be on your pinglist, and they get a notification (ping) when you post a thread of that topic. The number of people on your pinglist is the same as the number of FReeper names that you insert into the To: field when you're posting.
You're thinking of "post count", which is something completely different.
As for which pinglist is the largest, there are some pretty big ones, including the pro-military/Iraq/Afghanistan one, the "hold muh beer" one, and the Bush ones.