You'll need to edit addons/adminmod/config/adminmod.cfg. These are the settings you'll need to change:
Controls how many of the server's slots are reserved. This is useful only if reserve_type, below, is either 0 or 2.
This is the message shown to someone who gets kicked when there are only reserved slots left on the server, and they do not have reserved access.
This controls how reserve slots work on the server (the default is 0).
* reserve_type 0: Public slots are used in preference to reserved slots. Reserved slots are freed before public slots.
* reserve_type 1: One slot is always reserved (regardless of reserve_slots). If someone with reserve access joins into that slot, the highest pinger without reserve access is kicked to make room. Thus, one slot always remains free.
* reserve_type 2: Reserve slots are used in preference to public slots. Public slots are freed before reserved slots.
The difference between reserve_type 0 and reserve_type 2 may not be immediately apparent. Here's an example: Suppose there is a 16 player server, with 2 reserved slots. Thus, with no one on, there are 14 public slots, and 2 reserved slots:
* reserve_type 0: Someone with reserved access joins. There are now 13 public slots, and 2 reserved slots (the person does not take a reserved slot, because those are used only when necessary). The server later fills up, so there are 0 public slots and 0 reserved slots. If anyone leaves, whether they have reserved access or not, it is a reserved slot that is freed, not a public one.
* reserve_type 2: Someone with reserved access joins. There are now 14 public slots, and 1 reserved slot (the person takes a reserved slot). The server later fills up, so there are 0 public slots and 0 reserved slots. If someone with reserved access leaves, it frees up a reserved slot; if someone without reserved access leaves, it frees up a public slot.