I throw an executor but only one thread is executed

My question was that when trying to do a function with an executor (in this case, a multiplication of matrices) and when checking the thread that was in the run, it always came out the same, 12.

/*
Atributos de la clase
*/
    public int x,y;
    public  int matriz2[][];
    public  int matres[][];
    public int mattriz1[][];

    /*Constructor de la clase*/
    public prodMatConcurrente(int [][] mat1,int [][] mat2,int[][] matres,int x){
        this.x=x;
        this.mattriz1 = mat1;
        this.matriz2 = mat2;
        this.matres = matres;
    }



    public void run(){
       for(int i = 0;i<matriz2.length;i++){
            for(int j = 0;j<matriz2.length;j++){
                matres[x][i]=matres[x][i] + mattriz1[x][j] * matriz2[j][i];
            }
        }
        long threadId = prodMatConcurrente.currentThread().getId();
          System.out.println("I am thread " + threadId);
    }

    public static void main(String[] args) throws Exception{

        double tiempo_inicial = 0;
        int nNuc = Runtime.getRuntime().availableProcessors();
        float Cb = Float.parseFloat(args[0]);
        int tamPool = (int)(nNuc/(1-Cb));
        System.out.println("El numero de hilos es:" +tamPool);

        Scanner sc = new Scanner(System.in);
        Random rand = new Random();
        System.out.println("Introduzca la primera dimension de la matriz");
        int x = sc.nextInt();
        System.out.println("Introduzca la segunda dimension de la matriz");
        int y = sc.nextInt();
        int mat[][] = new int[x][y];
        int mat2[][] = new int[x] [y];
        int matres[][] = new int [x][y];
        //Inicializamos la matriz resultado
        for(int i = 0; i<x;i++){
            for(int j =0;j<y;j++){
                matres[i][j] = 0;
                }
            }
        //Rellenamos la matriz con random
      for(int i = 0; i<x;i++){
            for(int j =0;j<y;j++){
                mat[i][j] = rand.nextInt(10)+1;
                }
            }
       //imprime la primera matriz
        /*for(int i = 0;i<x;i++){
            for(int j = 0;j<y;j++){
                System.out.print(" "+mat[i][j]+" ");
            }
        System.out.println();
        }*/
        System.out.println("-----------------------------------");
        //Rellenamos la segunda matriz
         for(int i = 0; i<x;i++){
            for(int j = 0;j<y;j++){
                mat2[i][j] = rand.nextInt(10)+1;
            }
        }
        /*for(int i = 0;i<x;i++){
            for(int j = 0;j<y;j++){
                System.out.print(" "+mat2[i][j]+" ");
            }
        System.out.println();
        }*/
        System.out.println("-----------------------------------");
        prodMatConcurrente [][] h = new prodMatConcurrente[x][y];

        int tamPool2=Math.round(x/tamPool);
        tiempo_inicial = System.nanoTime();
        //Ejecutor que tiene de tamaño la dimensión de la fila de la matriz  
        //entre el resultado de la ecuación de Subramanian
        ExecutorService exe= Executors.newFixedThreadPool(tamPool2);
        for(int i=0;i<x;i++){
            exe.execute(new prodMatConcurrente(mat,mat2,matres,i));
        }


        exe.shutdown();
        while(!exe.isTerminated()){}
        //exe.awaitTermination(100,TimeUnit.NANOSECONDS);
        long finTiempo = System.nanoTime();
        System.out.println("El tiempo que ha tardado en ejecutrase es  "+(finTiempo-tiempo_inicial)/1.0e9);
        /*for(int i = 0;i<x;i++){
            for(int j = 0;j<y;j++){
                System.out.print(" "+matres[i][j]+" ");
            }
        System.out.println();
        }*/
    }   
}

The minimum number of threads you create when doing the Subramian equation with arg [0] = 0 is 8, so for an 8x8 matrix it would have to be 1 thread per row or so it should be.