Matthew Kenigsberg cole-h already got a tentative response:

We're tentatively looking at setting the GID of our group (nixbld) to 331 and the UID of our first build user (_nixbld1) to 331. Default installs create 32 build users, but Nix users can customize this at install time. Since Nix needs one build user for every concurrent build, we're trying to balance between the risk of low UIDs clashing with new macOS users in future updates, and still starting low enough that Nix users with Max/Ultra CPUs configure enough users to put that hardware to good use :)

We had a bit of a discussion, and this is probably the most expedient but the number being tossed around was 350.

I’m getting access to the bug again to see if anyone had any better advice.

Matthew Kenigsberg cole-h already got a tentative response:

We're tentatively looking at setting the GID of our group (nixbld) to 331 and the UID of our first build user (_nixbld1) to 331. Default installs create 32 build users, but Nix users can customize this at install time. Since Nix needs one build user for every concurrent build, we're trying to balance between the risk of low UIDs clashing with new macOS users in future updates, and still starting low enough that Nix users with Max/Ultra CPUs configure enough users to put that hardware to good use :)

We had a bit of a discussion, and this is probably the most expedient but the number being tossed around was 350.

I’m getting access to the bug again to see if anyone had any better advice.