Not trying to burst your bubble, but this game has been dead for months now. MW2 has had more players in it for weeks on end now.
Have you considered how many people of those 500 are on the DLC maps? Makes the pool to find players even less. Whenever the playercount in a certain gamemode goes under 1000, it get harder and harder to find a game to join on.
Imho, cut your losses and move on to a game that actually has people in playing it.
Matchmaking can be affected by a number of things. So of those reasons being the time or day that you play, your region, or the game modes you are playing.
To increase your chances of connecting to a game, try playing during peak hours on peak days. This is usually the weekend, or shortly after school/work until 10 or 11 pm.
Region can also play a factor in your ability to connect. If there are low populations of players around you, this can cause difficulty when searching for a game or higher pings when connecting to players further away.
The number of player in the specific game modes will also determine connecting to a game. As you've stated, playing in a game mode with a higher population is recommended. As a community member has pointed out, that number can also be divided by the number of players with or without DLC.
Try the steps out lined in the following link to set up your connection and improve your chances: Achieving Open NAT - port numbers included. Ghosts V2