Keep a fixed rack capacity

The paint rack holds ten cans even when some spaces are empty. Name that capacity once, then calculate the available spaces separately.

const int rack_capacity = 10; declares a value you cannot assign to later. The inner braces form a block: open_spots exists only inside those braces. Outside them, rack_capacity is still in scope, but open_spots is not.

#include <stdio.h>

int main(void) {
    const int rack_capacity = 10;
    int cans = 6;
    {
        int open_spots = rack_capacity - cans;
        printf("Open rack spots: %d\n", open_spots);
    }
    printf("Rack capacity: %d\n", rack_capacity);
    return 0;
}

Run the program and read both lines. Move the open_spots print below the inner closing brace to see the compiler's scope error, then move it back.