STNumInteriorRing (geometry Data Type)
Applies to: 
SQL Server
Azure SQL Database
Azure SQL Managed Instance
Returns the number of interior rings of a Polygongeometry instance.
Syntax
.STNumInteriorRing ( )
Return Types
SQL Server return type: int
CLR return type: SqlInt32
Remarks
This method returns null if the geometry instance is not a polygon.
Examples
The following example creates a Polygon instance and uses STNumInteriorRing() to find how many interior rings the instance has.
DECLARE @g geometry;
SET @g = geometry::STGeomFromText('POLYGON((0 0, 3 0, 3 3, 0 3, 0 0),(2 2, 2 1, 1 1, 1 2, 2 2))', 0);
SELECT @g.STNumInteriorRing();