The NixOS Foundation’s infrastructure team generates and uploads AMIs usually once per stable release.
For 19.09’s release, this ticket was created: https://github.com/NixOS/nixos-org-configurations/issues/90 and Amine sent this PR: ec2-amis.nix: add 19.09 amis by AmineChikhaoui · Pull Request #72186 · NixOS/nixpkgs · GitHub
We should update the release process documenattion (cc @disassembler) to include an issue to the nixos-org-configurations repo, for generating new AMIs.
Graham