In the current beta release of CM, we limit the number of customers we can display at one time to 500. So 500 is the current limit on the number of customers you could send an email to in one operation.
Having said that, if you have multiple pages of customers listed, each page having 500, you could repeat the process of sending that same email to each page of customers.
We realize that's not a very efficient method - longer term we plan to add email campaign features that remove these manual steps and make it possible to define which customers you want to have receive a specific email.
Regarding the management of email replies from your customer, we are still accepting requests and feedback before making decisions on a feature like that. We will direct any customers with similar requests to comment or follow on this post as well. Feedback helps us to learn how many customers are interested in each feature and prioritize what we should work on next.