This is the number of unique Subscribers you have. If a customer has multiple subscriptions, they are still considered as one subscriber. The Subscribers stat also excludes people currently on trials or anyone who has canceled their subscription.
Note: when clicking the stat, you will see a filtered list of your Subscriptions. Your Subscriptions list may not perfectly match the Subscribers stat for the reasons mentioned above. Subscribers a complex count that cant perfectly be represented by filters in a Subscriptions list.