It should be in the tag. I think it's called "Number plays".
Jim, the [Number Plays] tag is set to "Clear Only" edit type (and editing of that field is disabled).
It is really silly that the standard library fields are locked down like they are. Why can't we edit this? I can see how it can be useful to have things set this way for the novice. I can also see how you might want to limit editing these fields to people who really know what they are doing, because they are fields you use in other ways, so modifying them could potentially go so far as to cause MC to crash, or eat your files, or who knows what other terrible concequences.
However, there are so many other legitimate circumstances where the ability to unlock them and modify them (if only temporarily), would be quite useful. For example: this issue here, or the choices available for [Media Sub Type] whole dilemma.
Plus, there is already a "Revert all changes to stock fields" button. Why can't we have a UAC-like Administer Stock Fields button right next to it, which when pressed pops up an acknowledgment dialog that explains that this might "cook your girlfriend's bras in the microwave", and then if you're intrepid and click the "I'm Sure" button, it unlocks that one Tags' fields for that one session (and allows us to modify them).
look here: http://wiki.jrmediacenter.com/index.php/Changing_the_number_of_plays
That's a sneaky little hack you've got there, but it really shouldn't be necessary. I wonder... Have you checked to see if the Tags in the files themselves actually end up modified with the new data, or is it just a hack to get the data into MC's library?