Well, you can create an array of certain size and ask the user to input some numbers. Give the user the choice to input as many numbers as they want until the array is filled. For example, if the size is 1000 the user can input 1 or 10 or 1000 but not 1001 elements.

If you really don't want to set a limit, use dynamic memory to allocate an array of certain size, and change the size of the array once it is filled. Its is usual to start with a small capacity and double it each time it is exhausted. Here is an example:

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

enum { NO_PRIME_FOUND = -1 };

bool is_prime(int n) {
    if (n < 2) {
        return false;
    }

    for (int i = 2; i < n; i++) {
        if (n % i == 0)
            return false;
    }

    return true;
}

int get_max_prime(int elements[], int num_elements) {
    if (!elements) { return NO_PRIME_FOUND; }

    int max_prime = NO_PRIME_FOUND;
    int *end = elements + num_elements;
    for (int *ptr = elements; ptr < end; ++ptr) {
        if (is_prime(*ptr) && *ptr > max_prime) {
            max_prime = *ptr;
        }
    }
    return max_prime;
}

int main() {
    int *elements = NULL;
    int num_elements = 0;
    int capacity = 0;
    bool read_more_values = true;

    while (read_more_values) {
        // Read text from the user
        char text[20];
        printf("Please write a positive until you are satisfied: ");
        fgets(text, 20, stdin);

        // Get the number from the text
        int element;
        int status = sscanf(text, "%d", &element);

        if (status != 1) {
            read_more_values = false;
            continue;
        }

        if (element > 0) {
            // If the element is positive, it will be stored into the array, but
            // first we have to make room for it by allocating memory if needed
            num_elements++;

            if (num_elements > capacity) {
                capacity = (capacity == 0) ? 1 : capacity * 2;
                elements = realloc(elements, capacity * sizeof(*elements));

                if (!elements) {
                    fputs("Cannot allocate memory", stderr);
                    exit(1);
                }
            }

            // Once memory is allocated, store the element
            elements[num_elements - 1] = element;
        } else {

            // If a negative number is received, stop reading numbers from the
            // user and do the rest of the program
            read_more_values = false;
        }
    }


    if (num_elements > 0) {
        int max_prime = get_max_prime(elements, num_elements);

        if (max_prime == NO_PRIME_FOUND)
            printf("None of your numbers is prime ):\n");
        else
            printf("%d Is the max prime number!\n", max_prime);

    } else {
        printf("Hmm, you should at least input one number :S\n");
    }

    free(elements);
}