Re: TEST BENCH
Rather than the number of lines, what is more important is the complexity of your design. More the complexity, more the test cases you need. Testing all the possilbe condtions using Modelsim can be very cumbersome. If you write a test bench or multiple test benches to test the various conditions, it would be more convenient and efficient. With a test bench you can have a better control over your test cases. Just saying that you tested with Modelsim or Chipscope will not be a very good answer i suppose, since even for a very simple design writing a test bench is the most common verification procedure.
Even at a later stage if you have to do gate level simulation with the netlist generated by the synthesis tool, with slight modifications the same test bench can be used and you can cross verify your pre and post synthesis simulation results.