int argument = atoi(argv[1]);
FILE *fp;
char address[50]; // choose the size that suits your needs
sprintf (address, "/folder/item%dfile", argument);
fp = fopen(address, "r");
As you can read from the reference, a terminating null character is automatically appended to the string.
If you want to avoid buffer overflows, however, you should use snprintf
(which is shown in the same reference page as above), declaring the maximum size of the buffer so that the input doesn't overflow it.
int argument = atoi(argv[1]);
FILE *fp;
char address[50]; // choose the size that suits your needs
snprintf (address, 49, "/folder/item%dfile", argument); // 49 since we need one index for our null terminating character