Number of contacts in Klaviyo at a point in time

Hi @jonoroy,

Thanks for sharing this great question in the Community.

To start with, could I ask if something like a List growth report of your subscribers be beneficial? We have a specific built-in tool that provides you with the ability to do just that. 
For every list in Klaviyo, you can access a growth report that captures data regarding your list's growth over time. List growth reporting is only available for lists, and not for segments. To access your list growth report, head to your Lists & Segments tab and navigate into a given list. Then, click Reports > Growth Report. We have additional details and how to review the data in our guide on List Growth Reporting.