1. Using a tool like EMS SQL Manager, connect to the database and export the Users table in xls format.

Now you can see how many users was registered in a given period of time.

2. Soft delete the users from a given period of time:
In Host > SQL, execute the code:

update UserPortals set IsDeleted = 1
where CreatedDate > '2014-07-31'

Delete each time a group of 500-2000 users.

3. After each soft delete batch, hard delete the users using
Admin > User Accounts > Remove Deleted Users
( this can take several minutes)