At the moment, the retrigger action is restricted to retriggering the desired task 6 times , which means if someone needs to re-run a job 100 times, this will take either 100 button/keyboard mashes of Treeherder's retrigger button, or that person will need to submit 17 (100/6) custom retrigger actions maxed out at the 6 "times" limit.
The custom retrigger action method will end up spawning 17 action tasks.
The keyboard mashing method will end up spawning 100 action tasks (at the moment, bug 1521032 will batch those up and submit them with the "times" value maxed out as many times as it takes to submit all of the retrigger requests, so it'd end up with the 17 action tasks just like the custom retrigger action method above).
Either way, that's still a lot of action tasks to process. Action tasks are on much more limited numbers of workers than the actual jobs that will be triggered, so it's worth minimizing the number of action tasks that get spawned.
IIRC, the limit of 6 was chosen somewhat arbitrarily to stop people from requesting 1000+ retriggers, but I feel like 6 is unnecessarily arbitrarily low. It'd be much better if the "times" limit was higher. Something like 25, 50, or 100 seem like sane options that accomplish the same restrictions while not overloading the number of action tasks that get spawned. (jmaher also complained about the 6 limit, saying that's why he still goes for the button mash approach over going into the custom actions menu to request 6 retriggers 17 times over)