Good Morning, Patrick,
That setting is enforced right away during the Setup Assistant, so the only way to be able to cache more users would be to wipe the device and enroll it through a PreStage where that number is changed. It essentially partitions up the device during the Setup Assistant. I'll also throw some caution that the more users we have cached with the Shared iPads, the less storage each user would have.
I have seen an iPad get full with 6 users and it was only one user causing it all. And I am not even sure if it was for real or a bug in the system with that one user. Kids are in JK, so how much stuff could that kid possibly have???.
However, moving forward, I think iPads will be a great success in schools when ALL apps keep their data in the cloud. Either apples iCloud or the Apps own cloud. Already works like that for pictures and that is a GREAT thing.