You can use the Transaction Details API to get a list of transactions and then look up the details for each one, but unless you've tagged the transactions in some way that sets the recurring transactions apart from the initial transactions, you may have a hard time distinguishing them after the fact. What you really need to do is have your database store at least basic data, like for instance subscription ID's, start dates, and how many charges have been run for each one (that last value could be incremented from your Silent Post page - see the setting in the control panel) so you can later run the statistics you need.
You could, however, go to your control panel, click on active subscriptions and then on View All, and scan through them and get a pretty good idea of how things are going.
