Multiple surround effects can trigger. The phrase "You can't surround more than 1 Pokemon at a time" means just that. If I have two Pokemon in a line, adjacent to each other, they're not considered surrounded if my opponent has a Pokemon on each end of the line. If there's a Meowth switch, in a mass of Pokemon in 6-on-6, you could have a lot of KO's going on. I think I worked out the maximum one time, but I know you can get at least 5 Pokemon getting KO'd at once. You control the order of how your own Pokemon go to your Pokemon Center, if there's a simultaneous KO.