1) Download the Subscriptions export for "All" statuses and a Time range of "All Time."
2) Once downloaded, go to the activated_at column and remove all subscriptions that were activated after the last day of the month you are analyzing (if it's May 2020, it would 5/31/2020) since these will not be relevant to the month you're looking at.
3) Next, go to the expired_at column and remove all subscriptions that were expired before 5/31/2020 since these subscriptions would not count toward Active Subscribers for the month of May 2020.
4) Lastly, you would want to check the account_code for duplicates to see if any subscriptions are in there twice. Remove the 2nd subscription for each of these account_codes since they will only be counted once.