INT ST_NumInteriorRings(GEOMETRY geom);

SELECT ST_NumInteriorRings('POLYGON((0 0, 10 0, 10 6, 0 6, 0 0),
                                    (1 1, 2 1, 2 5, 1 5, 1 1),
                                    (8 5, 8 4, 9 4, 9 5, 8 5))');
-- Answer: 2

SELECT ST_NumInteriorRings('MULTIPOLYGON(
                                ((0 0, 10 0, 10 6, 0 6, 0 0),
                                ((1 1, 2 1, 2 5, 1 5, 1 1)),
                                ((8 5, 8 4, 9 4, 9 5, 8 5)))');
-- Answer: 0

SELECT ST_NumInteriorRings('MULTIPOLYGON(
                                ((0 0, 10 0, 10 6, 0 6, 0 0),
                                 (1 1, 2 1, 2 5, 1 5, 1 1)),
                                ((1 1, 2 1, 2 5, 1 5, 1 1)),
                                ((8 5, 8 4, 9 4, 9 5, 8 5)))');
-- Answer: 1

SELECT ST_NumInteriorRings(
     'GEOMETRYCOLLECTION(
        MULTIPOINT((4 4), (1 1), (1 0), (0 3)),
        LINESTRING(2 6, 6 2),
        POLYGON((1 2, 4 2, 4 6, 1 6, 1 2)))');
-- Answer: 0

SELECT ST_NumInteriorRings(
     'GEOMETRYCOLLECTION(
        MULTIPOINT((4 4), (1 1), (1 0), (0 3)),
        LINESTRING(2 6, 6 2),
        POLYGON((1 2, 4 2, 4 6, 1 6, 1 2),
                (2 4, 3 4, 3 5, 2 5, 2 4)))');
-- Answer: 1