I did some analysis of the mintage numbers for each year, and from memory (based purely on mintage numbers till 2008) there should be 3 * 72 in each $100 worth of 5c
However this does not take into account
-old coins have more time/chance of being removed from circulation
-1972s have been removed even more by collectors
-2009, 2010, 2011 and 2012 have been released making 72s more scarce
My guess is that you would be lucky to find 2 in each $100 of 5c (2000 coins)
Bottom line: You did well!!
PS. I use an led magnifying glass