Not at all. I was there for Mousefest and I think there were at least 500. But not everyone goes to the same park at the same time... It's a great time of the year to visit since it's not crowded at all during the first weeks of Dec.
To bad the illuminations party is only for those who book vacations through MouseEarsVacations.com...I'd love to join them but I'm DVC and arrange my own accomodations....wish others could opt to buy in..
I was at Mousefest last year, had a blast, and met tons of DISers. I hope to be there for it this year, too. Unless you stumble across or purposely go to one of the many meets, you probably won't notice any effect at all on crowd levels.