That depends on whether you're laying them side by side or stacking them one atop the other. I did some measurements and calculations of my own and lying side by side, it would take 84,480 pennies to reach one mile. Stacked flat one atop the other it would take 1,108,800 pennies to reach one mile.
A penny's diameter is .75 inches and a mile is 5,280 feet, which converts to 63360 inches, and I divided that by .75 inches and therefore, it takes 84,480 pennies to make a mile!
Well its a very interesting question and I luckily I have recently asked the same question from a friend of mine and she gave me the answer that there are 84,840 pennies in a mile! I agreed because she gave me a number of evidences.