How to specify the number of threads I want to use?

Question

I am trying to Multiply the matrix with use of threads. The code seems to be working. I just need to know how to specify the number of threads so that I can count how much time it's taking. I want to create a table of threads and time it's taking to compute the matrix. I am dynamically allocating the matrix and filling it with random numbers. I am creating threads to compute the resultant matrix.

C Program to multiply two matrix using pthreads without use of global variables

#include<stdio.h>
#include<pthread.h>
#include<unistd.h>
#include<stdlib.h>



//Each thread computes single element in the resultant matrix
void *mult(void* arg)
{
    int *data = (int *)arg;
    int k = 0, i = 0;
    
    int x = data[0];
    for (i = 1; i <= x; i++)
        k += data[i]*data[i+x];
    
    int *p = (int*)malloc(sizeof(int));
        *p = k;
    
//Used to terminate a thread and the return value is passed as a pointer
    pthread_exit(p);
}

//Driver code
int main()
{

    int i, j, k, row1, col1, row2, col2, r, sum;

    printf("Enter the number of rows for matrix 1\n");
    scanf("%d",&row1);
    printf("Enter the number of columns for matrix 1 \n");
    scanf("%d",&col1);
    printf("Enter the number of rows for matrix 2 \n");
    scanf("%d",&row2);
    printf("Enter the number of columns for matrix 2\n");
    scanf("%d",&col2);


    int **a = (int **) malloc(row1 * sizeof(int *));
    for(i=0;i<row1;i++)
        a[i] = (int *) malloc(col1 * sizeof(int));

    int **b = (int **) malloc(row2 * sizeof(int *));
    for(i=0;i<row2;i++)
        b[i] = (int *) malloc(col2 * sizeof(int));

    
        
    for(i=0;i<row1;i++)
    {
        for(j=0;j<col1;j++)
        {
            a[i][j] = (rand()%9) + 1;
        }
    }

    for(i=0;i<row2;i++)
    {
        for(j=0;j<col2;j++)
        {
            b[i][j] = (rand()%9) +1;
        }
    }
    
    int N = row1*col2;
    
    
    //declaring array of threads of size row1*col2  
    pthread_t *threads;
    threads = (pthread_t*)malloc(N*sizeof(pthread_t));
    
    int count = 0;
    int* data = NULL;
    for (i = 0; i < row1; i++)
        for (j = 0; j < col2; j++)
            {
                
            //storing row and column elements in data
            data = (int *)malloc((N)*sizeof(int));
            data[0] = col1;
    
            for (k = 0; k < col1; k++)
                data[k+1] = a[i][k];
    
            for (k = 0; k < row2; k++)
                data[k+col1+1] = b[k][j];
            
            //creating threads
                pthread_create(&threads[count++], NULL,
                            mult, (void*)(data));
                
                    }
    
    printf("RESULTANT MATRIX IS :- \n");
    for (i = 0; i < N; i++)
    {
    void *k;
    
    //Joining all threads and collecting return value
    pthread_join(threads[i], &k);
            
        
        int *p = (int *)k;
    printf("%d ",*p);
    if ((i + 1) % col2 == 0)
        printf("\n");
    }

    

return 0;
}