I never required a parent per child when I coached. Your question though has a lot to do with the group of students you have. If they are goin to require major wrangling or if you have a few that do easiest just to make it a requirement in my opinion.
The parents are indeed limited in what they can see day of tournament. I did still encourage parents to attend and most students had at least one present for at least part of the day. Once they saw it they seemed to enjoy themselves even if they could only watch the robot games. The students can still give the parents the blow by blow of what happened in each judging session. Which is what my team did. Also I would not plan as the coach to be the one doing all the wrangling. I actually had a parent responsible for watching the clock and wrangling after our first year. This lets you focus on coaching. Working with the students on what is next, etc. The schedule looks like you have all this time and sometimes you do but...most of the time as a coach I was doing something. Watching our time was just added stress and gave a parent a job. I also had the same parent double check that we had everything for the where we were going. Robot, project, etc. Again reduced stress and gave me a 3rd set of adult eyes that was getting us on the way to judging or robot game prepared. For the students I also gave them a job for each room and the robot game. They always brought the same thing (they still forgot but that was what the 3rd set of eyes would help us catch).