Update 1: 2018/10/26 4:06pm PDT
Sure enough, this 10,000-photo limit is formally codified, though it’s not specific to just Live Albums. In its Photos support documentation, Google confirms the 10,000 limit for private albums,
Live Albums rolled out to Google Photos a few weeks ago, but as users got excited and started creating their automated albums of people and pets, some of them noticed an upper limit that wasn’t exactly publicized: a Live Album can only contain 10,000 pics and no new ones will be added after that limit has been reached. Read More[Update: Confirmation + starting fresh] Google Photos Live Albums are limited to 10,000 photos, won’t add more after that was written by the awesome team at Android Police.