You will find a new feature on your UI once you refresh the page. This is Inventory Insights and can be found below Changelog on the left panel of the platform. Inventory Insights gives you an estimation of the available impressions to be served and the reach of unique devices. On the page you can set up a summary with the breakdown by geo (US states, cities, DMAs, congressional districts, and counties), app (app bundle and app name), or device information (make, model, OS); select time range and supply sources. The report can be downloaded to your PC as a CSV or XLSX file. 

Please pay attention that this feature covers CTV inventory only (connected devices and set-top boxes) without PC/phone traffic and US territory only.