Use the strlen() function provided by the C standard library string.h header file.

char name[7] = "Flavio";
strlen(name);

This function will return the length of a string as an integer value.

Working example:

#include <string.h>
#include <stdio.h>

int main(void) {
  char name[7] = "Flavio";
  int length = strlen(name);
  printf("Name length: %u", length);
}