Count Occurances of Duplicate Vales in a Large Excel File

I have 2 large data sets. Within each of them there is a field that contains street addresses. These addresses represent property owners. I need to sort the data so that I can find the property owners that own the most property. In a more technical sense, I need to sort the data according to the prevalence of recurring strings within a particular field of the data set. I want a new field in the data sets that counts the occurrences of the owners' address field so that i can sort the records according to. I would consider the data sets to be large, they are both more that 600000 records long and each record contains about 50 fields. I'm sure there is a very simple excel function that can handle this, I just don't know how to do it. Thank you