Have a look at the $N identifiers (/help $1-). $1 is the first word, $2 the second word etc.
Example:
Code:
; "&" is a wildcard for a word
on *:text:!countto &:#: {
  ; only count if the second word is a valid number
  if ($2 isnum 1-) {
    var %a = 1, %b = $2
    while (%a <= %b) {
      msg # %a
      inc %a
    }
  }
}