How do I get into several different files

Good night, I have a doubt, I have a program that will receive several files of type nome.txt , and I must count how many characters there are inside the file, I tried to make a prototype but the doubt is in how I will add this using% how do I get a file in txt and then check the characters ??

The following is the code below:

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

int main (void)
{
    int counter = 0;
    FILE* file = NULL;
    char * arquivo = NULL;
    char frases;
    size_t aux = 0;

    getline(&arquivo,&aux, stdin);

    file = fopen("arquivo", "r" );

    while(fgetc(file) != EOF)
        counter++;

    printf("%d", counter);

    fclose(file);

}