Your PS2 has 2 Ports for Joypads. With our 4-Player Adapter you can make 5 Ports or even if you buy 2 adapters you get the maximum of 8 Joypad-Ports. There are some games for 8 Players.
The adapter fits on every PS2, the old one or the newer slim version. It depends on the game if you have to connect the adapter to Joyport 1 or 2. Maybe you just have to try both.