Kazunori Akiyama, Mariafelicia De Laurentis, Ziri Younsi, Yuto Akiyama, Dominic W. Pesce, Geoffrey C. Bower, Kazuhiro Hada, Jens Kauffmann, Shoko Koyama, Kotaro Moriyama, Derek-Ward Thompson

The Event Horizon Telescope (EHT) has directly resolved and imaged two supermassive black holes and opening a new window on black hole physics. However, the current array is limited to only these two brightest nearby targets. This white paper outlines how future EHT upgrades, anchored by the Atacama Large Aperture Submillimeter Telescope (AtLAST), will enable a transformative expansion of the accessible population of event-horizon-scale sources. By substantially improving sensitivity and multi-frequency capabilities, EHT+AtLAST will enable demographic studies of black hole growth, accretion physics, and jet launching across a wide range of masses, environments, and accretion states.