When setting up a new subscription, one can set values for "Number of issues to display to staff" and "Number of issues to display to the public." If you don't set these in a specific subscription, then they fall back to the values in the system preferences StaffSerialIssueDisplayCount and OPACSerialIssueDisplayCount, respectively. It'd be good to add a note along the lines of "If unset, will default to the value in StaffSerialIssueDisplayCount (X)," where X is the value from the syspref.
Maybe just, Default, if unset: X? Thinking something nice and short.
*** This bug has been marked as a duplicate of bug 26549 ***