Something

on *:JOIN:#: {

  ;read the first wb for the user
  %wb = $readini($mircdirWBs.ini,$nick,1)
  if(%wb) {
    var %i = 1
    ;calculate how many WBs the user has
    while (%wb) {
      inc %i
      %wb = $readini($mircdirWBs.ini,$nick,%i)
    }
    %i = %i - 1
    %x = $rand(1,%i)
    %wb = $readini($mircdirWBs.ini,$nick,%x)
    /msg $chan -WB $nick - %wb
  }

}