Heads up, Creators: On Dec 13-14 you may see a noticeable decrease in your sub count as we remove spam subscriptions. If spam is removed, you'll see a YouTube Studio alert: https://goo.gl/qGDBvX This should help give you confidence that the subs you do have are real fans!
We use technology to identify spam accounts and we don't remove legit subscribers from the sub count. On December 13-14, 2018, we've removed spam subscribers and hence you see a noticeable decrease in your subs. You will see an alert in YT Studio if impacted with more info.