RE: CRM 2016 notification email
Hello Nuhr,
Actually just like Iswarya said, you can use "Wait until conditions" in custom workflow.
You should also create a special field that increases its value with every case for customer. When counter hits definied value then condition is fullfilled.
And, what's more important, you will need another workflow for zeroing field "case counter" every month.
This can be hard, because there's no option to wait "for end of current month" in OOB workflows. If you don't want to dive into codeing, just create workflow on demand, that sets "record counter" to 0 and set reminder for 1st day of every month :)
But if you do not want to solve this in that way, you will have to create plugin.