--index is the position within the role, not the row number in the file. Filter to the admins, number them from zero: two admins means index 0 and index 1, and the fourth row of the file is one of those two.
It is the same number that names the cached state, which is the easiest way to see it:
the number in the file name is the position within the role
bash
sdods auth list -p rwa-bank -e local
If you want all of them rather than one, skip the counting:
bash
sdods auth capture -p rwa-bank -e local -u admin --all
One more thing that had us puzzled for a while: capture skips a user whose cached state is still fresh, so re-running it can look like it did nothing. --force recaptures regardless, and auth.maxAgeMinutes in the project yaml is what "fresh" means.