I understand 5 to 7 thousand people where in attendance last year.  Check out the Top Truck Challenge Power Wagon.    link, http://www.pirate4x4.com/forum/showthread.php?t=681448  

yep...a couple buddies of mine are going this weekend.just got their rig running good so look for a tan nissan patrol going toppless in the mud. it was jacked up last year w/44" swampers on it and they were too bald for this year so they downsized a little bit on the tire size...should be fun!  

here's another link to a site with info, http://www.mudallica.com/