Display count of invoices on customer form

I have searched the forums and found solutions that are similar, but not exactly what we are looking for.  Also, I am not sure if this is something that should be done by saved search or if there is an easier solution.

I would like a field on our customer form that indicates the count of invoices for that customer.  We would be able to use that field as a criteria in workflows for a lot of our internal processes.  For instance we want to assign certain tasks to salespeople when a customer has ordered three times.

Thanks!