CRM Online - Field to count marketing list number of members, for static and dynamic list, with more than 50000 members

Hello all,

I have to create a field on marketing list to count the number of members. This field needs to count the members for both types of marketing list : static and dynamic. A marketing list can contain more than 50 000 members. 

How can i do that? 

I was recommended to use fetchxml with javascript, or create a workflow, but i don't know how to do this.

Thank you!