Sort work item query results by the number of subscribers?

In just looking at the attributes one can include in a query -- probably no way.

If you can update the project area there may be a way to keep track using a calculated value script. I haven't tried this but you may be able add an attribute "subscriberCount" to the Type, make it dependent upon Subscribed By, then create a calculated value script that set the value to the number of subscribers.  To update past items, you could make the count dependent on another attribute (e.g., a another dummy attribute) and do a bulk change on the other attribute to trigger the subscriberCount calculation.

I wrote a script that walks all of the open work items in a category hierarchy and writes a .csv file showing the number of number of subscribers, votes, id, and summary.