That one will be a lot harder than you think. We had a member that wanted to have a school canteen monitored for sound levels and a sign light up saying "Hush" when it exceeded a certain level. He had all the code working and just had to adjust the threshold value. After six months he reported back that it was not possible.
I think the problem was that the signal the microphone picked up was more dependant on the distance to the microphone rather than the actual level of noise.
Also I have never seen even a professional piece of equipment to count people successfully unless they were restricted by a turnstile.
You have chosen two difficult things to do.