1. Create a Number field on Account named Number of Attachments
2. Start Rollup Creation Process
3. Choose Account as your target obejct
4. Choose Number of Attachments as your target field
5. Choose Attachments as your child object
6. Choose Deleted as your field(This field is used because it is guaranteed to be populated)
7. Choose Count as your Rollup Type
Save everything and your are done!