sizeofを使っています。
#include <stdio.h>
int main(void){
char filename[][256] = {"aaa.txt","bbb.txt","ccc.txt"};
int i;
int count = 0;
FILE *rfp;
count = sizeof(filename) / sizeof(filename[0]);
for( i = 0; i < count; i++ ){
printf("%d\n",count);
if *1 == NULL ) {
fprintf(stderr,"ERROR");
}
}
}