Alright, let's say you have a 24 slot server and you want to reserve slots for your admin's or vip's.
At first, consider that vipslot need's one slot for itself (this slot is hidden) to let the vip's connect to the server if the server is full.
Now you must set cvar publicslot and maxplayer to 23 (not 24).
To adjust the diffrence, you can host 25 slot's and set both cvar's to 24 or you will have a 23 slot server.
If you want to keep the vip slot's free then you must set publicslot to 15 e.g.
That means only 15 non vip's can join the server and all the rest of the slots are reserved for vip's (this is not recommended, use the first method).
The hidden slot acts as a kind of buffer if your server is full everyone can try to connect to the server over this slot
but it will only pass vip's and kick a public by random after it. Because of that, sometimes you might see 25/24 slot's.