If you're not able to alter the number of registrations on your classroom, this could be because of two situations:
1. You may not have enough licenses allotted. As you can see on the upper-left side of the image, the number of registrations exceeds the amount they have allotted:
If this is your case, you may ask your school's License Manager to allot more licenses to you in order to increase the number of registrations in that classroom.
2. You may be trying to add a number that's inferior to one of your current amount of students in that classroom. You can see this here in the upper right corner:

In this box, you're meant to place the total number of students you want in that classroom. Taking this image as an example, if what you wanted was to add 12 students more to that classroom, you'd add up 13+12, that'd be 25, so in that box, you'd place the number 25.
If neither of those solutions solves your situation, please, don't hesitate to contact us.