In Advanced Find you won´t be able to sum up the number of activities, but you can export as pivot excel file and then sum the number of activities for each account.
Or you can add a report for this.
Another way would be to use a custom field and the help of workflows starting for each related activity incrementing the field on each account. Then you can add this column in your advanced find and will see the number of activities for each account. But
keep performance in mind using workflows for this.
Carsten Groth http://carstengroth.wordpress.com Microsoft Dynamics Certified Technology Specialist