MySQL:

ST_NUMGEOMETRIES

ST_NumGeometries(gc)

Returns the number of geometries in the GeometryCollection value gc.

ST_NumGeometries() handles its arguments as described in the
introduction to this section.

URL: https://dev.mysql.com/doc/refman/8.0/en/gis-geometrycollection-property-functions.html

Example

mysql> SET @gc = 'GeometryCollection(Point(1 1),LineString(2 2, 3 3))';
mysql> SELECT ST_NumGeometries(ST_GeomFromText(@gc));
+----------------------------------------+
| ST_NumGeometries(ST_GeomFromText(@gc)) |
+----------------------------------------+
| 2 |
+----------------------------------------+