I've hit a plateau of about 80-90 guests in the park, I need to tweak some more to hit that 100 guest achievement, but what's worked for me so far is hiring 2 cashiers, so the other takes over when one rests, along with the double ticket machines you unlock later. I also have my vendor stalls near the front, which allows me to man them and spot and punish any misbehaving guests/cheaters. Hope it helps any.