我检查了你的代码，删除了不需要的内容。在输入和排序中，你似乎做了两次。我试图记录我所做的更改，并解释为什么要做这些更改。

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

// It's generally a good idea to use #define when you have some global constants
// I made some of the constants larger than what you showed to prevent issues
#define MAX_NAME_LEN 40
#define MAX_SURNAME_LEN 40
#define NUM_PEOPLE 10
#define BUFF_LEN 100

// Separate your struct...
struct person {
    int num;
    char name[MAX_NAME_LEN];
    char surname [MAX_SURNAME_LEN];
};

// ... and array decleration
static struct person people[NUM_PEOPLE];

// I added this function, to make it easier to display a person
void print_person (const struct person * p) {
    printf("Person %d %s %s\n", p->num, p->name, p->surname);
}

// This function will print out every person in the people array
void print_people (void) {
    for (int i=0; i<NUM_PEOPLE; i++) {
        print_person(&people[i]);
    }
}

// Compares two people by number
int compare_people_num (const void * a, const void * b) {
    struct person * p0 = (struct person *) a;
    struct person * p1 = (struct person *) b;
    return p0->num - p1->num;
}

// Compares two people by name
int compare_people_name (const void * a, const void * b) {
    struct person * p0 = (struct person *) a;
    struct person * p1 = (struct person *) b;
    return strcmp(p0->name, p1->name);
}

int main (void) {
    int i;
    char buffer[BUFF_LEN];
    for (i=0; i<NUM_PEOPLE; i++) {
        printf("Enter number, surname, and name of person %d: ", i+1);
        fflush(stdout); // fflush makes sure that our text is shown to the user

        fgets(buffer, BUFF_LEN, stdin); // Read user input in to buffer

        // It's unclear what you were doing here
        // This sscanf line takes a line of text, and splits it into a number and two words
        // It then stores that number in people[i].num, and stores the words in name and surname
        // However, right after this, you have several while loops that appear to be manually doing the same
        // thing all over again. If you want to read all of the input in, just the line below is enough
        sscanf(buffer, "%d %s %s", &people[i].num, people[i].name, people[i].surname);
    }

    // We've read all of the people in now
    // Uncomment the next line to check out the output at this state:
    // print_people();

    // To count names, we first need to sort the people by their name
    // We do this using a qsort call
    qsort(people, NUM_PEOPLE, sizeof(struct person), compare_people_name);
    // Once the names are sorted, we'll calculate how many different names there are
    // We start the count at 1, and start checking from the second person (index 1)
    // This is because the first person will always be unqiue, and we can't compare to
    // person negative 1
    int n_names = 1;
    for (i=1; i<NUM_PEOPLE; i++) {
        char * current = people[i].name;
        char * previous = people[i-1].name;
        if (!strcmp(current, previous)) {
            n_names ++;
        }
    }

    // Now we have to sort the people based on their num field
    // Again, in your code, it looked like you were doing this more than nessecary
    // We just have to call qsort once, as such
    qsort(people, NUM_PEOPLE, sizeof(struct person), compare_people_num);

    // We will also do a loop through to calculate the average surname length
    float avg_surname_len = 0;
    for (i=0; i<NUM_PEOPLE; i++) {
        avg_surname_len += (float)strlen(people[i].surname);
    }
    avg_surname_len /= (float)NUM_PEOPLE;

    // We're all done! The people are sorted by number.
    print_people();
    printf("There are %d unique names\n", n_names);
    printf("The average surnames is %f characters\n", avg_surname_len);
}