How to count the number of times a string appears in a column?
 

I have an excel worksheet with, among other things, a column in which each
cell has a list of alpha numeric strings like, "NA998748". The column is
several hundred rows long, and each cell contains up to 10 or 15 strings. I
need to be able to run a function that tells me how many times each exact
string appears in the column, which exact one appears most, etc etc. What is
the best way to do this?