Then, you were kinda close, but it should then be something like this:
if (getObjects(Slenderman.class).size()<6)
{
livescounter--;
if (livescounter == 0)
{
Greenfoot.stop();
return;
}
else
{
addObject(new Slenderman(), getWidth()-1, Greenfoot.getRandomNumber(getHeight()));
}
}

