I've known this for a while but didn't want people to abuse it, since its Treyarch and Gameservers though... 
Buy the lowest slot unranked server you can.. then ad this dvar to your server CFG..
sv_maxclients "24"
I'm paying for a 15 slot atm and get 24 slots, not sure if it can go higher or not, I havent tried, pretty sure it could go to 32 atleast though, In all honesty if you use this and want to keep using it, don't go over 24 as they will probably catch on.

Buy the lowest slot unranked server you can.. then ad this dvar to your server CFG..
sv_maxclients "24"
I'm paying for a 15 slot atm and get 24 slots, not sure if it can go higher or not, I havent tried, pretty sure it could go to 32 atleast though, In all honesty if you use this and want to keep using it, don't go over 24 as they will probably catch on.