Which UID/GID is free for new package

After reading https://unix.stackexchange.com/a/222376/246754 How rpm ensure that no uid/gid clash with each other ? What I am looking is similar to https://github.com/NixOS/nixpkgs/blob/68196a61c26748d3e53a6803de3d2f8c69f27831/nixos/modules/config/users-groups.nix#L640-L642 (given for comparison) I have though to building a VM and take the /etc/passwd to see taken uid. It doesn't fully anser my need because 174 is not their by default. Again nix also reserve some uid like 0 for root https://github.com/NixOS/nixpkgs/commit/dba1d48b7897c49e5df9430c6b60f17e03ec361e and I can see the list without building a VM or can query with the cli. They also have an allocator for id so I don't have to manage it. This is a scriptlet run when rebuilding /etc/passwd PS: I have tagged rpm-spec because the linked question is about RPM but the problem is more likely to happen to other tools like apt, yum, pacman, brew... If you have knowledge for other system please create another question and answer it (to keep Q&A style) or comment