i as row_num,

case $2

when 1 then jsonb_build_object('a', i)

when 2 then jsonb_build_object('a', jsonb_build_object('b', i))

when 3 then jsonb_build_object('a', jsonb_build_object('b', jsonb_build_object('c', i)))

else jsonb_build_object('a', i)

end as nested

from generate_series(1, $1) as i;