One, about 2/3 through 2003. We were in pretty bad need of one. It did a few things, really. It cleared out unused characters (I kept everything played in the last month or two I think; those characters were cleared but had passwords and other stuff like info preserved), made some of the trash leave, and served to rebalance the playing field. It seemed that a lot of people were starting to reach RL100+. Also, it wouldn't be fair to instate my EXP system with old characters. It would be unfair to the new characters due to the old ones getting "easy exp" or "easy remorts".
I don't regret it. It's not that you have to worry about them all the time on my server. They don't happen with great frequency... we've had one in two years.