[WebMethod]
public PolContainer PolizaXRiesgo(string polAliasRiesgo)
    {
//SQL Query using (SqlCommand cmd = new SqlCommand("SELECT * FROM ...
        foreach (DataRow dr in dt.Rows)
        {
        pc.Poliza = new PolizasRes();
        pc.Poliza.Id = 7541;
        pc.Poliza.NumPoliza = "FA56779057";

        pc.Poliza = new PolizasRes();
        pc.Poliza.Id = 9327;
        pc.Poliza.NumPoliza = "FA70049057";
        }
    }

public class PolContainer
{
    public PolizasRes Poliza { get; set; }
}

public class PolizasRes
{
    public int Id { get; set; }
    public string NumPoliza { get; set; }
}

<PolizaXRiesgoResult>
   <Poliza>
      <Id>9327</Id>
      <NumPoliza>FA70049057</NumPoliza>
   </Poliza>
</PolizaXRiesgoResult>

<PolizaXRiesgoResult>
   <Poliza1>
     <Id>7541</Id>
     <NumPoliza>FA56779057</NumPoliza>
   </Poliza1>
   <Poliza2>
      <Id>9327</Id>
      <NumPoliza>FA70049057</NumPoliza>
   </Poliza2>
    (.....)
 </PolizaXRiesgoResult>

public class PolContainer
{
    public PolizasRes Poliza1 { get; set; }
    public PolizasRes Poliza2 { get; set; }
    public PolizasRes Poliza3 { get; set; }
}