You can run vacuum verbose on just the postgres database and get the
global numbers at the end. gotta be a superuser as well.
# \c postgres postgres
postgres=# vacuum verbose;
.... lots deleted.
DETAIL: A total of 7664 page slots are in use (including overhead).
7664 page slots are required to track all free space.
Current limits are: 1004800 page slots, 5000 relations, using 6426 kB.