On systemd systems the UID boundaries are not user configuration… – Info Linux

From looking at the systemd source, it appears to be setting a build config option SYSTEM_UID_MAX from the build machines /etc/login.defs SYS_UID_MAX value (with default 999 if missing) — so the max is being set at compile time.

That right? Here is the code from the meson config file:

system_uid_max = get_option('system-uid-max')
if system_uid_max == -1
        system_uid_max = run_command(
                awk,
                '/^s*SYS_UID_MAXs+/ { uid=$2 } END { print uid }',
                '/etc/login.defs').stdout().strip()
        if system_uid_max == ''
               system_uid_max = 999
        else
                system_uid_max = system_uid_max.to_int()
        endif
endif

In the fallback case, it just shelling out an Awk job to look at /etc/login.defs. As the main case, that would be wrong. Assuming that it is right to get this info from some login.defs, you have to look at that login.defs which is in the target sysroot, not the build machine’s own login.defs.

(And that means that the file has to exist in the target sysroot, so in a distro build setting, the build of systemd has to have a dependency on whatever package installs that file into that sysroot.)