Go into one of the tables (tabs) and click settings. Then click relationships. You should see one connecting Customers to Activities. On the Customers side you want to click Create Summary field. Additionally you can enter a criteria to filter that number if you're looking for a specific type of activity count.
Now that you have the field created you can go to the Customers table, click reports and charts and create a new one. Choose the customs you want to see, maybe Name, Address, Phone, # of Activities for example.
Hope this helps! Good luck!