tl;dr: ATProto doesn't have "instances" like Mastodon because it deliberately decouples hosting from apps—your data lives in swappable personal hosting (a PDS), while apps merely aggregate over everyone's data, similar to how RSS feeds relate to readers like Google Reader. This avoids Mastodon's fiefdom problem where your identity is tied to a specific server, federation can break between admins, and your account dies if the instance goes down. Decentralization in ATProto is measured by hosting portability and app diversity, not instance count.