And for completeness, a Ruby implementation:

  puts STDIN.read
    .split(/[^A-Za-z]+/)
    .group_by(&:downcase).transform_values(&:size)
    .sort_by(&:last).reverse
    .take(ARGV[0].to_i)
    .map{ |l| l.join("\t") }.join("\n")

Wordier than the bash script, but still pretty short and fairly trivial. Anyone could make this in five minutes.