First, it's not possible to tell how many leads have a currently associated Munchkin session, since if you clear cookies in your browser Marketo will not know that you aren't capable of logging activities under that cookie in the future. In addition, cookies expire after 2 years of inactivity, and that can't be logged in the Marketo db, either.
If you do a Smart List with Anonymous IP [is not empty] that approximates the number of known people who have had an associated cookie at some point.
For more detail, you can extract the Cookies field using the API and parse it out to get the earliest date each person created a session that eventually became associated.
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