Code:
  
on $1:TEXT:/rolls? (\d+)/:#room: {
  var %Roll.NumberOfdice = $regml(1)
  var %Success.Count = 0
  var %Trait.1 = 0
  var %Trait.2 = 0
  var %Trait.3 = 0
  var %Trait.4 = 0
  var %i = 1
  while (%i <= %Roll.NumberOfdice) {
    var %Die.Result = $rand(1,6)
    if (%Die.Result == 6) { inc %Success.Count 2 }
    if (%Die.Result == 5) { inc %Success.Count }
    if (%Die.Result == 4) { inc %Trait.4 }
    if (%Die.Result == 3) {
      inc %Trait.4
      inc %Trait.3
    }
    if (%Die.Result == 2) {
      inc %Trait.4
      inc %Trait.3
      inc %Trait.2
    }
    if (%Die.Result == 1) {
      inc %Trait.4
      inc %Trait.3
      inc %Trait.2
      inc %Trait.1
    }
    var %Roll.Result = %Roll.Result %Die.Result
    inc %i
  }
  msg $chan * $nick rolls %Roll.NumberOfdice dice ( %Roll.Result ) for %Success.Count success(es).
  msg $chan * $nick may activate an Anima Trait of 4 to reroll %Trait.4 dice.
  msg $chan * $nick may activate an Anima Trait of 3 to reroll %Trait.3 dice.
  msg $chan * $nick may activate an Anima Trait of 2 to reroll %Trait.2 dice.
  msg $chan * $nick may activate an Anima Trait of 1 to reroll %Trait.1 dice.
}


event triggers when in a line 'roll <number>' or 'rolls <number>'
occurs.
The $regml(1) identifier replaces your $2 which is the dice-number.