@list.each_with_index do |item, idx|
  if((idx + 1) % 6 == 0)
    # Insert ad partial
  end
  # Print out each object in the list
end

> list = [1,2,3]
#⇒ [1, 2, 3]
> list[1...1] = 0 # insert at position 1
> list
#⇒ [1, 0, 2, 3]