Answers for "how to make an array that can be any length c"

size of an array c

int a[20];
int length;
length = sizeof(a) / sizeof(int);

passing the length of an int array c

//I suggest using strlen (of #include<string.h>)
//if you want only the size of the occupied index

int prices[5] = { 1, 2, 3, 4, 5 };
int size = strlen((char*)prices); // convert it to string 
printf("%d", size+1); // Just add 1 that's it