In order to calculate how many exactly documents you have, you will have to figure out two variables. First you will need to know how many inches of paper you have. Secondly, you will have to figure out how much paper you have in an inch. Once you have both of those numbers just multiply them out and that will give you a great idea of how many documents you have.
To figure out your sheets per inch number just look inside your containers and count how many files you have in one foot. Then you will have to count how many sheets are in your typical file. Just open up one or two files and physically count how many sheets are in each file. Average that number out and multiply the number of documents in that one foot section times the average file length and that will give the approximate number of documents you have per foot. Multiply that number by the total lineal feet of container space you have.