I've setup a custom field to add +1 each time Activity is Logged.
One challenge is filtering activity history to only count activities that are outbound attempts vs. activities that were just logged by systems.
It's tough to come up with logic using things like subject contains that is bullet proof.
There is no "Type" constraint on Marketo filter "Activity was Logged"
It would be great if Marketo could add that to the list of available constraints.