Is it possible to change the total worm count in a game (default with all teams 4x4) maybe in local.xml or lvlsetup?:p

It's possible, but since 4 worms is like the standard, you'd have to find every single "Worm Count" relative option and mod it for six or more, and there's a lot of files to go through. Then you'd have to get a map in which spawns 4+ player-controlled worms.

The worm manager overflows if you try to spawn more than 16 worms, making the game crash if I remember correctly.