Just to clarify, max_input_vars is a PHP configuration option which can't be changed from within the Moodle interface - you'd need to get a server admin to change it. You can, however, check what it's currently set to through the Moodle interface by going to "PHP info" in the Server section of Site Administration - that'll give you a huge amount of info about the server's PHP configuration, including what max_input_vars is set to.
The workaround I mentioned is a part of Moodle 2.3 and is not something you need to do anything with - it just automatically reduces the number of students shown per page if max_input_vars would be exceeded. It does this to make sure that you'll be able to successfully save all grades entered, as otherwise PHP will effectively cut off the form when it hits max_input_vars, resulting in only the first part of the grader report saving. I was therefore suggesting it as a possible explanation for why you're only seeing a couple of students, not as a solution to it.
If you can get your server admin to increase max_input_vars (maybe start by doubling it) and you subsequently get more students appearing, that would provide confirmation that this is what's happening - and you can either choose to live with seeing fewer students per page in the grader report in courses with lots of grade items, or you can get your server admin to increase max_input_vars even further (but I'll leave that to you and your server admin to decide!).
Hope that all makes sense,