Okay, now i found some time to check again, how to enable multitap in PSX, so that you can play with more than 2 Players simultaniously.
And here is what i done:
Go to RetroArch Quick Menu by pressing Hotkey + B (on Playstation-Controller B = X) while in game.
Then go to Options and change "Pad 1, 2, 3 etc. Type" from "default" to "standard" and Multitap 1 to "enabled".
Restart Game and now it should work with more than 2 Players.