The Action Center is designed for you to keep tabs on customers you may want to follow up with. Think of the Action Center as your one-stop shop for checking Mail and Ping, while also helping you to keep track of customers you recently spoke with.
New customers are defined as those that have had their first-ever contact with you within the past 30 days. This corresponds with the "NEW" label next to users shown on the My Customers lists. To change the number of new customers you see in your Action Center, remove customers from your New Customers list and add them to a different list of your choosing.
You may also update the number of customers showing up in the Action Center for you to follow up with by opening each individual Mail or Ping sent to you by your customers. You do not need to follow up with a customer through Mail or Ping in order to update the number of customers displaying here.