如何在C中具有动态线程数

我想使用线程在c中计算数学求和。（∑x ^ i，从0到N）每个线程应计算求和项的每个项，最后在程序中，程序应将所有所有项求和并打印出来。我应该如何动态地设置线程数？这是我的代码：

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

pthread_t sadaf[10];
int i,a[10];
long int x,N;
int sum=0;
pthread_mutex_t mtx;

void *Myfun(void *tid)
{
    int *ThreadID=(int *)tid;
    pthread_mutex_lock(&mtx);
    printf("The thread with id of %d calculated x^i\n",*ThreadID);
    a[*ThreadID]=pow(x,*ThreadID);
    sum=sum+a[*ThreadID];
    pthread_mutex_unlock(&mtx);

}

int main()
{
     int d[10] = {0};
     printf("->**************************************************************************<-\n");
     printf("This program will calculate the following function:\n-> ∑x^i ,From 0 to N \n");
     printf("->**************************************************************************<-\n");
     printf("Please enter x:\n");
     scanf("%ld",&x);
     printf("Please enter N:\n");
     scanf("%ld",&N);

    for (i=0; i<N; i++)
    {
        d[i] = i;
        pthread_create(&sadaf[i],NULL,Myfun,(void *)&d[i]);
    }
    for (i=0; i<N; i++)
    {
        pthread_join(sadaf[i],NULL);
    }


    printf("The sum is: %d\n",sum);

}