Check out my post below, specifically item #1.
It is my understanding that the 'subscribe' activity is tracked in the analytics database as a WATCH activity type in the jivedw_activity_fact table. Unfortunately, and you will see this in my post, there appears to be a bug that is causing the WATCH activities not to be propagated into the analytics database. Mark Williams mentions that a fix is already in place and gives the release numbers.
When that fix is present, you can determine the number of users that have chosen to WATCH (250) any event by joining jivedw_activity_fact to jivedw_user on user_id and doing a count. You may need to watch out for people that choose to REMOVE_WATCH (260) if you are trying to get a current snapshot of everyone that is watching.
Hope that helps.